I know this is late to the party, but I wanted to say thanks for the hint about the crossover switch. I’ve been playing TF all this time thinking crossover switches weren’t supported! Turns out I needed to drag the rail just a tiny bit more than I had been
Also, I suggest another way of thinking about the signalling: A signal is a place a train can stop at. Consider that the train draws a path from where it’s at to where it wants to go. If that path conflicts with another train, then the train will proceed as far as the signal immediately before the conflict. If there is no intervening signal, then the train won’t move.
There’s three reasons why you won’t want to place signals everywhere. 1. Cost. 2. Looks and acts messy and 3. (the most important one): you don’t want a train to stop where it can block (or foul) an intersection. So, don’t place any signals at least a train length after an interesection. So, just before the insection is okay, and recommended, UNLESS doing so would also block the previous intersection.
A train will ignore the “back” of a signal, unless that signal is marked as “one-way” (click it and select “one-way”). In this case it will not even consider that direction as a valid path.