In this mode, you can play entirely without opponents or financial pressure.
All locomotives from the chosen era are available to you from the very
beginning, and you have unlimited funds.
Trains always use the shortest available path even if a slightly longer route
would be faster.
If you have more then one train to a station configure the used platform manually (edit route > click station) to divide them optimally between the available platforms. Hovering with the mouse over a platform shows you how much (in percent) the platform is currently in use. If you have trains driving through a station don't use that platform for other trains as you can't select on which platform trains drive through.
Everywhere where you use shunts place entry-signals from all directions facing into the area.
Don't place signals if a full length train does not fit behind it.
(Tip: a train is ~14km long compared to track length in building mode)
Supply Towers and stations have built-in signals
Signals "think" ahead if a train reserving a path to them will cause a conflict somewhen further ahead. (if it wouldn't do that for example, you couldn't use automatic block points on single tracks or trains would soon block each other head on head). Think of it like trains can pre-reserve a path ahead of these signals, even if that path is still in use by another train, as long as it won't cause a conflict in the future.
A signal in the opposite direction of a train has no effect.
It acts like there would be no signal at all.