Partsman, This is quite a subjective subject but.......
N gauge locos take about 1/5 amp on full load, quite a number of HO locos need 1 Amp on full load and since you may want to run 3 locos MU'd together you will have to cater for three times the max load per loco.
What is known as "equipment wire" is often used and it is generally stranded.
16 strands of 0.2mm copper conductors can carry 3 amps and has an overal diameter of 1.6 mm. This is what I use for everything
However, the wire itself has resistance and long runs (ie 20 feet or more) will deliver a voltage to your track less than your throttle output voltage if the current is above roughly 1 1/2 Amps.
Tips:-
1) Use stranded wire, it is more flexible and breaks are less likely.
2) Use a heavier gauge of wire rated at 6 Amps for large layouts.
3) Some modellers like to run track feed "busbars" under the baseboard following the route of the track and tapping into these busbars to feed the track at regular distances.
4) Do draw a circuit diagram of your wiring and label it..... You have no idea how much effort this can save you in 5 yrs time when you have all sorts of wiring added to your layout and problems occur.
All the foregoing is for single throttle or block wiring with multiple throttles.
I believe DCC controlled layouts have much simpler wiring but the wire sizes indicated above is still valid.
No doubt, other members will advise in this thread based on their experience, no opinion is ever wasted if you are aware of the fundamentals of the loads you are trying to supply and how far from the controller the most distant connection is to the track.
(Just my two British pennies worth)
Errol