Variety

If you've got a number of these stretches a bit of variety will probably be best.

A 10-20' radius curve in N scale looks awesome, by the way.  I'd recommend using a single big curve in at least 1 location.

Tangents

I would suggest the answer lies in what part of the country and the feel that is trying to be conveyed.

In some parts of the country the railroad can have tangents that are dozens of miles long.  Other parts of the country the railroads are curvier than a snake's rump.

The first part of this video was shot out of the back of a gas electric doodlebug running along my prototype:

The portion from 0:48 to about 2:15 was shot on the railroad I model.  Obviously, I will be minimizing tangents.

On the other hand if I was modeling in the upper Midwest or the plains, I might want to emphasize the tangents.

In any case, I wouldn't want the track exactly parallel to the fascia, I would want it at a shallow angle to improve the look.  I think that curves are scenically more interesting because they give multiple angle views of the trains.  

Angled or Undulating

I would suggest the answer

That's a biggie.  Think David Barrows CM&SF RR with long stretches of straight mimicking the region he was modeling.  OTOH, D&RGW didn't have much straight tangent on much of their mainline thru the Rockies.

Curves

Nothing like watching a train weave it’s way through a bunch of curves, especially the dreaded S curve.

"What are folks

  What do you see when looking at the prototype? Do you most notice curves, or grades , or tangents, or what? Differnt railroad leave different impressions. Modeling Tehachapi I'd go for curves and steady grades while in the valley below I'd go for straight as an arrow paralleling the the fascia as the highway does the railroad for 100 miles or so to the north....DaveB

Mix by terrain

Thanks.  This is all very helpful.  I have looked at the maps for the prototype, and I can adjust to straighter/slightly curving sections on one level matching the minimal curving on the prototype and put in more undulations on another level where the prototype curves a lot more. 

The discussion helped me to think about why the tracks would or wouldn't curve.  And when I started thinking that way and now rechecked the maps, that matches what the maps show.  Where the area is roughly flat with little or no obstacles, the track was mostly straight.  Where hills and lakes take over, it curves around them.  That will help with my scenic work when the time comes because there will be natural reasons for those undulations.

Thanks,

-Ed

variation

As a variation, a curve could be put in place for a reason, the railroad crosses a stream or river at an angle that would not make sense with straight track. The track can curve to a more suitable crossing angle and then resume back to the straight. This would be where the water is running nearly parallel to the track but make a jog that causes the railroad to cross it. The tourist railroad I volunteer on has a couple of spots like this. You could also have a train enter mid way back and then gently curve all the way to the back and pass behind a farm or other such thing just for visual interest. Just make sure you can easily reach a derailed train because it is a sure thing that if it derails that will be where it happens despite you best efforts otherwise

Depends on many things...

Hi Ed, all

Your answer depends on many things. In the real world, the result of straight or curved track is an interaction of history and available technology. 

Some aspects of history:

• Land ownership - If your railroad was one of the early ones, the route could be affected by land ownership and the politics attached to it. If a landowner had a enough "clout" to resist the railroad coming through their land, then the railroad would find another way to get through with a deviation around the "no-go-there" land.
• Geography - ideally trains run best if the grade is flat or favours downhill the route of most loaded trains. If the geography is not ideal, the track can be deviated away from the straight line path between 2 points so as to optimise train performance, especially if the performance of the locomotives of the time is limited. 
  
  However tight curves following the terrain can easily add significantly to the effective grade resistance faced by your trains, resulting in reduced hauling capacity in the days of smaller locomotives. 
  
  Don't be fooled by what the eye perceives as flat. Sometimes what appears flat may be tilted and may not actually be "horizontally flat".
• Bridges and tunnels - cost big money, so the route would be generally planned to minimise the length of costly items like bridges. That means ideally the track crosses waterways perpendicularly where possible. Dodgy foundations and a whole flock of other reasons might cause deviation from this design policy, but the ideal approach would be to jump these obstacles in as short a distance as possible, even if that means slightly deviating the tracks with curves and a slightly longer track to get the correct approach angle. 

Some aspects of available technology

• If labour is cheap and plentiful and the available technology is shovels and wheelbarrows, then cutting through a hill to maintain a straight alignment may be the preferred construction option.
• If labour is scarce and costly and the available technology is shovels and wheelbarrows, then going around a hill rather than through it gets your tracks further quicker and cheaper, overturning the advantages of a straight alignment at the cost of increasing your operating costs when compared to the straight route. 
• If modern technology is available (Bulldozers, jumbo drill rigs for drilling basting holes, etc), then it may make more sense to amortise the higher construction costs when compared to cheaper running costs of trains that can haul more on a straighter flatter track profile.
• longer bridges with an extra span or two came into their own once concrete supports and steel plate girder or truss bridges became available. Again making a straighter track alignment possible
• Available locomotives - The same track worked by small steam locomotives, operating at their maximum performance when the track was built, would not be a problem for multiple diesel electric locomotive lash-ups available today. 

   How much of this applies to your design is up to you, but you can justify straights or curves as you see fit.

Prefer the Look Of Curves

When designing my plan using the valuation maps I obtained from the historical society, it was apparent that there were long stretches of tangent track most everywhere - both in the towns and countryside.  It is western Ohio after all. Occasionally there would be a big sweeping curve to help alter the direction, but these were few and far between.  With selective compression ruling the scaling down of the plan and selective towns / cities to include, I just opted to place most every stretch of track, including those going thru metropolitan areas on long, gentle sweeping curves.  For me it's all about aesthetics and that is avoiding as many straight lines as possible.  To each his own and Rule #1 always applies.