There is a cure for the jumpy 5v start up; diodes on the output to the track.
+5volt . --->l---->l----- +3.5v
+5volt(same) -----l<----l<---- +3.5v (same...connect ends)
A general purpose diode ("a one way check valve") will eat at least /about .75 v . So 2 or 3 inline eats about 1.5v-2.25v.DC. ....For AC, since it's power moves both directions, you need double the diodes, 4 or 6 diodes, faced to opposite each other and connected, + to - & - to + (arrows/lines above) to form an inline bi-dirrection string. The string can go on one output (if equipped with 2+) or just as often at the common to lower multiple tracks/acc.s..
Bridge rectifiers are just 4 diodes in one package and can be an alternative choice. Maybe cheaper and more common with heavier amps. Overating on amps gives a cooler mod (the power loss =heat) over on voltage is good for spike resistance. Rate to the max transformer output. (Parraleling diodes for increasing amps doesn't work well IMO. They tend to fail that way. I'd run two of a same batch/same brand parrallel of I was barely over min tolerance though.)
Now you'd start at 3.5v-2.75v (etc) while the transformer is at 5v output. Your max voltage will be shy the same amount fyi.
This trick , plus removal of diodes temporarily is one way of getting AC with a an offset wave output (+ > -) or (- > +). Lionel uses that offset with a special DC only relay for blowing whistles.
Especially where a linear motor is used (solinoid coil and plunger... Reverse unit?), DC can more likely cause the plunger to become magnetised and stick to a steel strike pad/stop. It happens with AC too, and DC usually don't use an e-unit, but an AC engine run on DC might be a different story as the fields are more constant.
And back to cooling, AC coils are expecting the cooling break. They run hotter and pull harder on DC.
This cooling can apply to motors too fyi. That would a matter of case by case as to which way is "best". (Just like old Williams Pittman's on half wave vs full wave AC: heat/power at X-speed via timing of coils is a balancing act.) (Half wave is like bumpy DC/ or one direction of diodes on AC...or a major offset thats skipping one wave 100%) . The point is that you may get more heat on DC, you may get less, but with heat, DC can be more brutal to equipment in ways