In most cases, layout design decisions are neither inherently good or bad…… as long as the layout owner is crystal clear as to the pros and cons of that decision. Therein often lies the problem. Occasionally we look back and realize, “well it “seemed” like a good decision at the time. Now that I see what the ramifications are, the whole picture, I wish I’d thought that through a little more.”
Whether to go with a double track main line, as opposed to single track, is one such decision. It’s a big enough one that the future layout owner owes it to themselves to make sure they’ve thought it through well ahead of time.
I would categorize many of my customers as “rail fans”. Formal, more involved operating sessions, sound a bit too much like the work of the day jobs they are trying to escape from. For this reason, a double track main holds enormous appeal as they visualize multiple trains sliding through scenery without any mental energy being required to avoid cornfield meets. That indeed is an advantage of double tracking. If you’re a prototype modeler replicating a line that had double track main then, well, you are going to need those two tracks!
However, there are no free lunches and when it comes to double tracking there are significant aspects you need to consider before jumping in blind. Let’s look at the other side of the coin now, the cons of double tracking:
Double tracking involves twice as much track to lay. Seems obvious but often the magnitude doesn’t sink in. You’ll be paying for twice as much product and expending more than twice the time to get it down. Momentum in the early stages of a project is critical to maintaining enthusiasm, especially for those new to the hobby. The extra effort associated with laying that extra track will slow momentum a bit
Extra crossovers and turnouts. You’ll need to have a means of crossing from one track to the other. This means the insertion of crossovers along the line. More cost, and more labor complexity. In addition, each crossover eats up about two feet of main line run (20 inches to be exact with number 6 switches) which will impact the design of small to medium size layouts.
Track separation. In order to keep trains from sideswiping one another, attention will need to be given to keeping the tracks properly separated. This is particularly an issue on curves where the separation needs to be increased to account for car overhang. In HO, I generally suggest a two-inch parallel track separation on the straight runs and two and a half inch separation on curves.
Decreased layout depth. Because you will need that two-inch track separation for a double main, that chews up two inches of layout depth that could be used for scenery and structures.
Bridges. Many commercial bridges and viaducts are set up for single track only. Are you prepared to kitbash/modify them for double track?
Loss of operational complexity. This can be a pro or con. A single track main requires sidings in order for trains to pass one another, a feature that adds operational interest to many modelers. You lose that operational interest when you double track.
So, if you understand the costs of double tracking, and can embark on it with your eyes wide open, go for it!