I believe most of the time

I believe most of the time it's simply a patter of how many containers are on the train, and where they're going. I often see mixes of one and two stacked containter, and ywo 20 footers under a single 40. I don't recall ever seeing 20 footers bulktainers (liquid) stacked, though... or even 20 foot flatracks stacked, though I'm sure they could be.

I found out it also has to do

I found out it also has to do with the weight of whatever is in the container, and the route it's taking, avoiding lower bridges and obstructions, and the yard of origin, and destination, as far as what equipment they had for handling the containers.

Mike.

Clearance

NS / PAS (Pan Am Southern) have an interesting “fillet/toupee” operation in Mechanicville, NY to deal with clearance further east to Ayer.  Intermodal arrives double stacked from points west, and gets “filleted” - double stacks turned into single stacks.  Pan Am takes the single stack trains from there to the intermodal terminal at Ayer, MA, and the return trip.  Westbound trains are “toupeed” in Mechanicville to combine the single stacks to double stacks.

not due to height. as single

not due to height. as single in a bathtub styled car, and many other cars in same train double stacked. even the regular tin can car.  not seen any haz-mat placards on them. weight? in the usa cannot be over X amount due to movements by truck. and come out of the same load up line, double or single stack, heck i have seen semi truck vans in the bathtub cars.

maybe just a top heavy load in the can? but some time lots of them in one train.

i believe a rule about the 20 foot tank, can not be top loaded. (moving liquid can cause a tip over, derailment, etc). is there even a 40 foot tank?

most of what i see is along hwy 99 in the central valley of ca. in and out of the bay area, (both hwys 80 and 580) over donner pass, points north to oregon.  both up, and santa fe / burlington.

so no restricted hight problems within some hundreds of miles...farther?

several reasons

Single stacked

Cars are single stacked for clearance.  Somewhere on the route there is low bridge or tunnel that won't clear a double stack.  If that's the case all the wells will be single stacked on the car.

There are rare cases that empty boxes will be single stacked to avoid blow over conditions.  In the western US the winds blow strong enough to blow over a stack car with two empty cans double stacked, so empty containers are single stacked.

It could be they just didn't have enough containers to double stack each car.  For train handling purposes it is better to single stack some platforms rather than let a platform be empty.  If I have a 5 well car and only 8 containers, its better to go double-single-single-double-double than double-double-double-double-empty.

20 ft boxes are always put on the bottom when mixed with 40+ft containers.  Always.  Longer boxes only have connectors at the 40 ft marks and do NOT have connectors in the middle.  It you put a 20 ft container on top of a 40 ft container there would be no way to secure the inboard end of the 20 ft container.

Blocking Reasons

Having worked an terminal that originates and terminates container trains, the reasons listed above are valid, but are missing one other reason: blocking. Most often single stacked cans where domestic cans, not international cans that I saw. The difference in destination blocking was the reason for single stacking vs double stacking. I would say 90% of the time single stacks would be either weight or destination blocking. Only 10% would be single stacked for clearance. Remember the big trains from the west coast to the Chicago area is huge business for the railroads. Chicago has lots of different container yards depending on final location. It is very plausible that a single stacked can in Seattle will get double stacked in Chicago before going to New York or other points east.

Here is what I can think of as a complete list of reasons:

Weight

Clearance (solid blocks of single cans vs intermixed)

Destination (blocking)

I would say that if you see single cans intermixed with double stacked cans it's likely due to blocking reasons rather than anything else. A solid train of single stacks would indicate clearance issues.

yes for a "string" of single

yes for a "string" of single stack in well cars it does make sense they are going to be top loaded down the road before heading east.

as to weight. from what i have read, no mix of two 20's with one 40 or longer on top, a 40 top & bottom, 40 bottom 53 top. is going to outdo the weight rating on any container car. except for a "special" extra legal load in the can.

as to clearances. maybe up north in was state, but all of the west is good to go for even double high cubes. heck even the subway in downtown reno has more than enough room.

Topping off

I don't know how plausible that would be.  Maybe it depends on era or a specific train or railroad.  In Chicago they tended to either run through or be completely grounded, then drayed to interchange.    Top loading would be a really slow process, It would be much quicker to double stack the cars, then pick up a pre-loaded block of cars than to bring in a train and top load it.  How do you know in Seattle on Monday what the gate is going to be on Wednesday in Chicago?  The ramps where I worked, never "topped off" cars.

i do not even begin to say i

i do not even begin to say i know anything about road bridges back east. aka everything east of say the mississippi river.

but one would tend to believe with the trillions of dollars to be had each year via containers, all the low bridges would have been rebuilt by now. as even so many of them are through shipped over the alantic to the "old world".  pending... could be faster by rail through the USA, than the canal,(or ship too big),  or round the cape.

i do know some containers are off loaded at one port, say san diego, or even mexico, then rail shipped elsewhere due to costs. or shipped to the last port of call for that ship to then be loaded, again costs. as lots of trains north-south bound, loaded or just nothing but empty's. but then in my world most everything does this if just to get to a mt pass out of ca. (only 5 ways in or out, one north to oregon,  two east -reno-, one sorta east and south to barstow, last is the coast rt to la).

Bridge => easy, Tunnel => not so easy

The clearance problem on Pan Am Southern is the Hoosic Tunnel, which can currently clear auto racks but not double stacks.  They got that clearance from the original by lowering the track and grinding off the tunnel roof.  They'll need to do more of that to get double stacks through.  But it's almost 5 miles of tunnel, so that's not cheap or easy.  ( https://en.wikipedia.org/wiki/Hoosac_Tunnel)

Low Bridge, everybody down

While that is technically the Erie Canal, which was more or less put out of business by the railroads, in most cases they don't "raise the bridges", they lower the track.  There are also several locations, primarily tunnels on the east coast where the track can't be lowered and its not feasible to raise the tunnel. 

Also not all containers are the same height, especially domestic ones, it's possible that a route will take two 8 ft  containers but not two 9 ft containers or even an 8 and a 9 ft container.

bridge  => easy, tunnel =>

bridge  => easy, tunnel => not so easy.

only takes money to do anything.

and as much money that is at stake in container traffic. it would be cheap n easy to bore out a whole new tunnel.

i do not know this area of this tunnel, but if a rail road can cut off even 10 miles each trip. or reduce a grade by 1% for 10 miles. or straighten out a area. each of these would save a lot of money over the years.

I guess you better get a hold

I guess you better get a hold of the Pan Am Southern, and let them know what's up!

Let us know what you find out.

Mike.

Bridge - not so easy, Tunnel harder

Tunnels are tricky because you can't just dig lower or cut higher, some tunnels are lined and if you cut higher, you destabilize the roof and risk collapse of the tunnel.  Same with cutting through the floor of the tunnel.  If its under water there are the risks of leaks.

I don't see how one can say that bridges are "easy".  Completely rebuilding a through truss bridge is pretty darn expensive.  I could see where somebody is assuming "bridges' means a low overpass, and thinks its just shimming up the bridge, but about half the problem is that the bridge members on the portal of a through truss  are too low or there are diagonal braces on the interior that intrude into the clearance plate of the double stacks.  Those require structural changes to the bridge design and are very expensive or require a complete replacement of the bridge.