Signalling depends largely on the speed and density of traffic on a line.Usually a line signalled with Semaphores (known as Absolute Block Signalling in UK terms) would have about 2 Miles between the last Signal (the Starter) controlled from one box and the first Signal (the Distant) controlled by the next box. On a lightly used line such a Hertford North - Stevenage (which I used to work trains over) with one train per hour , Two Aspect ( Red/Green and Yellow/Green) would suffice.On a busier line with trains every 20 minutes Three Aspect Colour Lights (Red/Yellow/Green} would be used and on a high speed + density line such as the East Coast Main Line Four Aspect Colour Lights (Red/Yellow/Yellow/Green) would be used with flashing Yellows to indicate a junction ahead.On a very lightly used line (say Goods Only) its possible no signals except for Fixed Distants.Its a complicated Subject!
Problem is Trainz signal system does not work exactly like real life, and The AI thinks slightly different to every thing known to man or beast,
I would normaly place signals around stations/yard exits/entrances to give an apperance correct, this would be suplemented with invisible signals to help the AI ( it has 2 left feet at the best of times) now after leaving the station area an going out into the wilds it depends if the line is multi track or single track because both need to be treated different, with single track there I would have nothing, unless there was a passing loop this way you will not get the AI sending to units head on, in real life this would not be the case, as the driver would not get the clear to leave if there was something coming the otherway, back in days of old this was done with sticks/battons, the driver would be given one before he left the station and he would hand it in at the next station, it would be returned by the next driver going the other way, but with the telephone this changed.
anyway with trainz I go according to the ammount of traffic I intend to put down the line, but normaly about 3/4 times the length of the longest unit likly to transit, this keeps the flow going, and stops units hogging switches long before the are going to arrive and blocking other units, it also tends to give better control over unit speed, and better breaking to stop units over running the red because the were going to fast.
As I say if you are cab driving no problem, unless there are AI units also in the region, but for DCC and AI control you need a little sideways thought towards the signaling system you impliment, but I have never seen the Proto system that workes in Trainz correct under all conditions.
but then its sometimes fun to see the mess up the AI can make, and some time you just whant to pull your hair out as it will be different each time.