outdoor electric trains NOT battery rf controlled

[boppa]

HO locomotives, and their circuitry, are designed to operate at no more than 18 volts DC. I'm thinking any significantly higher voltage will result in higher current, and PPHHHITTTTTTTT!.........you let "the smoke out", the loco goes up in flames.

HO outdoor, will require first, a very stable roadbed to lay track on. The subroadbed will have to be weatherproof (homasote is totally out, as is cork). And, finally the track will have to be cleaned, well,( and dried to prevent shorting ) before operation, and "the hand of God push starts", will, pretty much, always be part of the operations.
The larger the outdoor part, the greater the maintenance that will be required. Is it possible ?, absolutely! The drawback will be the required maintenance, and the acceptance of occasional problems. (and that's without any knowledge of the kind of weather your friend normally has.( HO cars don't handle wind all that well )
As for track and structures, a thorough research of G scale products "materials" would be in order. Some plastics do not do well in an "exposed" environment.
I'm not saying "don't do it", but if he does, I wish him luck.

thats the beauty of using a pwm control system..

it never `sees' a voltage above 12v

but the actual peak voltage can be in the hundreds of volts quite safely

(the trick here is that PWM bit (pulse width modulation)

its fancy blackbox bits that allow much higher supply voltages to be used without higher currents being drawn..

so no smoke escapes!!!

this is a good thing!!

(higher peak voltages also help overcome high resistance track connections as well)

its widely used by almost everyone here (well all the dcc crew anyway)- its how dcc turns the constant 12-20v track voltage into the `slow/med/fast' motor voltages that the loco motor uses...