Good question...

I'm interested in hearing the answer as well... if I were to "guess", I would say creosote. (it acts as a wood preservative against elements, and stops critters from chewing on the wood)

But that's just a guess on my part. (it's been 40 years, but I can still smell that smell of creosote we used to have to paint all of our feeder boxes with... eek... nasty smell)

same as the sides

That very much depends...

Both of these depend on era and prototype.

From everything I have researched and know about wood cars, "namely boxcars" underframes were typically painted to protect the wood. Wood cars were not typically treated and had shorter intended life spans. Usually wood was plentiful and would be replaced as needed. Furthermore, much higher quality wood was available. I have read sources where they used 2x10 solid oak for roof walks!!!

Roof walks were usually painted. 

Most of all look to the prototype. There are probably 101 different ways to build a wood boxcar beyond what I have described.

Paint it all

All wood was painted on rolling stock as a way to preserve the wood and slow the process of deterioration. THe car color was the typical color of the underbody but some companies applied a dark asphaltum, as has been noted here.

I read somewhere that running boards were sprinkled with sand or grit as the paint dried. I wouldn't worry about modeling that kind of detail but various planks in the running board could display different paint tone as they were not all replaced at once. Some planks may be a weathered gray as they were not painted when installed or they might be a different shade of the color on the car. Here's an example.

The tape on the car sides was to convey a newer paint look after the car was weathered. This would indicate replacement boards or a paint out of original lettering. You can see the final result on this Pennsy USRA single-sheathed boxcar on this blog post.

I don't worry about applying an exact match to a car color. Weathering is the great leveler and makes each model and important scenic element on an operating layout.  Here's another blog post sharing details on several boxcar weathering jobs.

Eric

Was this before anti-climbers?

Re:  creosote - the proper application would make the wood insanely heavy for a vehicle!

There's also plenty of safety issues that got handled in this era.  The wood cars of that era were hazards on wheels... prior designs were worse.. telescoping in derailments, for one.  Thus, underframes were constantly being redesigned, until they got to steel.

The thought about what was used is a good question for historical research, and plenty would be available in contemporary safety-issue reports and the then-evolving design standards.

In Madison, NJ, Rod's Steak House has a pair of FEC Parlor cars in use as dining cars.  They recently underwent renovation (unfortunately not a restoration) because they were in need of maintenance, and as a restaurant, needed to conform to fire code that altered (actually, only slightly) some details.  But their experience would probably unveil some construction techniques and materials.    (on the wall, they have a letter from Lucius Beebe : https://en.wikipedia.org/wiki/Lucius_Beebe)  These cars were of truss-rod construction, which is another prominent detail of the era.

So, a proper scene in that era might include demolished rolling stock that displays the subsequently banned construction materials and techniques. Either on RIP track or as trackside wreckage.

Heavy Creosote?

The creosote we used on the farm must have been thinned somehow, because it was honestly no heavier than any paint on coating would be. We painted it on with brushes... about three coats. I don't remember it being any heavier than the show feeders, which only used paint.

Paint

Varies from railroad to railroad, but generally the undersides were painted the same as the top.  Some roads used an asphalt type coating, some did not.  Most painted roofwalks.  Brakemen wore hobnail boots for traction.  There were no crosswalks from the roof walk to the ladders so the brakemen just walked on the roof at the corners.

Creosote and other things

Creosote was used as a preservative on railroad ties. It was not used in boxcar construction. Many boxcars were used to haul grain. Creosote would soak through the boards and contaminate the load. Paint would also not stick to creosote. 

And before we add more comments about car construction, it's important to note the era you are posting about. I model 1926 and my comments were focused on cars of that era. From the details I've found over the years, the same comments apply to thousands of freight cars built in the Teens, too. 

The OP noted 1900+ but that is quite vague. Sure, there were LOTS of wood underframe boxcars working the rails in 1900. Steel underframes and/or centersills started being installed on new boxcars starting about 1902 with the Pennsy XL class cars. The Pennsy built more than 37,000 of these cars in the next decade. Other lines added similar cars in the Oughts. Steel underframes and centersills enabled longer trains to be handled as locomotives grew in size and capacity. By 1910 very few new boxcars were being built with all wood underframes. By 1915 the requirement was in place for all boxcars to have steel or steel-framed ends. Many railroads started installing steel centersills on boxcars. Many retained the original truss rods or just the truss rods closest to the side sills, which were still wood. Wood draft sills (centersills) were banned from interchange in 1928.

Please be aware of the incremental changes in railroad car construction and safety appliances in the 1900-1930 decades. A blanket statement that "wood cars of that era were hazards on wheels" is not helpful in understanding the technological evolution. No years were noted in that statement. If it came from White's "American Freight Cars" then it might not apply to many years beyond 1905 as his work in that book ends when steel construction and underframes came into popular use. 

Here's a look at some of the interchange rules up to 1930.

1870 - archbar trucks replace wood beam as US standard
1893 - Safety Appliance Act passed (brakes and couplers)
1900 August - all cars required to be equipped with air brakes & knuckle couplers
1909  - MCB drops recommendation for "Air Brake" stencil
1909 - MCB standard MCB-26A to locate reporting marks at left end of car side 
1911 - Safety Appliance Act amended (ladders, grabs,etc.)
1911 - United States Safety Appliances Standard stencil introduced 
1914 (or 1916?)  - Steel or steel-framed ends required or recommended on box cars
1915 (approx.) - All cars to be equipped with compliant ladders, grabs, etc.
1920 Oct 20 - ARA issues standards for stenciling reporting marks
1925 - United States Safety Appliances Standard stencil requirement discontinued 
1925 Jan 1 - ARA revised dimensional data stencil standards in effect
1925 May 1 - nominal capacity & load limit stencils required
1927 April 7 - load limit star stencil recommended, when limit is not bearings
1928 - wood draft sills banned from interchange

Many of these are important modeling details if you model the Teens or Twenties. It's a rewarding challenge.

Eric

@Eric Thanks for the summary

the summary of regulations and standards.  

The reason I 'broadened' the discussion was not to lose focus, but to include details that were still common on the rails.

Just because someone models (say) 1910, doesn't mean that 1910 era cars are the only  appropriate vehicles.  They would do well to model 30-40 years prior to 1910.  The Safety appliance act made some of the transition more sudden, but these 'tools' were add-ons, not part of the framework of the cars.

BTW, nobody here mentioned Broad Gauge (6 ft.) - until now,  sorry.

And, yes... it is a challenge. 

Generalities

I would caution using general statements. If you model 1910, you would need an array of freight cars built in the previous twenty years and possibly a few from the 1880s. but anything older would be in rough shape. The national freight car fleet grew in those 40 years, too. The oldest cars would be removed from service through attrition. They would be more prone to in-service failure when coupled with more modern cars that were built after 1902 when steel frames became more common. I suspect there were very few examples of 1870s era 24-foot boxcars that were in-service circa 1910. 

When modeling any era, a goal of mixing in older and modern cars will make your layout better reflect the prototype. Being aware of regulations and practices is a solid step towards that goal. Let the prototype be your guide.

Eric

" I suspect there were very

 Yeah, even if they were not worn out they likely wouldn't be worth updating the couplers and brakes to meet post 1900 era regulations. A lot of change occurred  around the turn of the century .....DaveB

Thanks

I asked the question because I have been going through my car collection most of which is good for the early 50's. I have though always appreciated the look of early freight cars which I thought came from around 1900. From Eric's comments though I suspect that they actually represent pre 1900. The cars in question are the old timer, 36ft, MDC cars, most with truss rods. Some though do have early steel under frames. Previously I have just painted them black and called it good. So I asked the question to see what would come back. More then I expected which I appreciate.

Jack

MDC cars

Most MDC "old time" cars represent cars from the early 1900's that have updated safety appliances and are closer to a 1920 era car than a 1900 era car.  If you have the old MDC cars with the separate grab irons they are more typical of the 1895-1905 era cars.

Following

Lot of good information in this thread.

As others have said, the answers really depend on the year and the railroad. Things changed over time, and regionally, and at the whims of regime changes at car shops.

Roofwalks were usually painted the same as the car body, for example. But not always. Some railroads, such as the Denver & Rio Grande, specified that roofwalks were to remain unpainted to provide traction for the brakemen, since linseed oil paints get slippery when wet. Other railroads specified that sand was to be mixed into the paint used on roofwalks.

Underframes were often painted the same as the body. But not always. Pay attention to good photos of truss rod cars and you will notice here and there that the paint only extends as far back as a man with a brush could reach while standing to the side of the car. Many 19th century cars never received paint on the underframe because the logic was that the roof and carbody was enough protection from water, and even though steam and compressed air spray painting was developed in the late 1880s, hand-painted wood freight cars with brushes continued to be the norm into the 1930s.

I have read a lot of paint instructions for a lot of railroads while helping to research the 19th railroad paint color compendium, and there was a belief that black paint, usually made from either a bitumen or asphaltum pigment, and later carbon black, was a better preservative for metal parts. Because the truss rods and  kingposts are usually metal, those would usually be painted black. This practice phased out around the 1920s as paint quality increased and steel cars proved that there was no real difference between bitumen and metallic pigments for metal preservation.

The 36-foot boxcar was introduced in the mid-1880s but did not become common until the 1890s. Again as mentioned, the MDC model represents a car with the post-1900 safety appliance standards, so is perfect for 1910s-1930s era layouts, but not so much for 19th century applications. Freight car technology developed rapidly between 1870 and 1910, so many cars had very short lives as they were quickly phased out due to changes in regulations, in Master Car-Builders Association recommendations, and in capacity needs. When the Safety Appliance Act fully went into effect, most railroads just pushed the old cars off the rails and burned them rather than go through the effort of upgrading them. It was cheaper just to buy new, compliant equipment, so by 1910 you really don't see many 19th century cars left in interchange service at all.

For those interested in 1880s-early 1900s paint information, the compendium can be found here: https://docs.google.com/document/d/1jbXGfg9V3cY0awKamkrwM2JlLSQepapDwU9A3Dx43-M/edit

We are working on transferring it to PacificNG for easier access as well, which is an ongoing project: https://pacificng.com/wiki/index.php?title=Historic_Railroad_Paint_Color_Index