Dc fed to an ac board really shouldn't do anything bad unless it was an awful design. Every ac cycle has a + and - wave; dc is either a + or - wave (a wall of water until turned off if you will; from a battery, or pulsed + or -, not true dc.)
Now some logic circuits are known to use ac , split the dc+ and dc- and use them for separate system sections. (Walthers O SD-¿??) So something might not work right on dc, but it should live.
Run ac to a dc only motor or unrectified board and you cook it. (most can motors & magnets for the field vs coil field)
The only exception I can envision might be setting it into a write mode or dump by accident.
Most trains will need the volume off or older, whistle motor switch added.
Smoke unit elements will perform differently, but again, the smoke unitsboard wont care either.
A diode is a one way gate. A bridge rectifier is a package of four diode aimed to form a 4 door corral. Ac goes in, dc pulses come out. One of the arrangements is often found on our boards, but there are some exceptions.
Learn to find them and use diodes and BRs. They are very reliable solid state items, not complicated logic like command can have.
A capacitor ,similar to a battery, vs filter/disc type, can smooth pulses to smooth and quiet things if you want. Fry something? Try fuses For the price of a handful of diodes, br, led and resitor variety packs... oh well. (not that hard)
Ac is wild horses and given access to two corral doors so they can come and go as needed , 60 crossings per second, 30 each (+/-) direction.
In the corral one remaining gate keeps entry going head first only, inline into a high wall stall (dc+) and loops around and dumps at the remaining one way gate letting them back into the corral where they will choose the easiest path to follow at that second.
"The 4 diodes" can be tapped off in other ways too. My point is more about what to look for before you test ac on a board or motor. Motor fields & board rectification.
98.8 of ac items dont care much about dc. Id mind the motor and goodies for heat, but otherwise pretty fearlessly.
Some folk hated the buzzing some eunits have, and clacking E-units. The intro of sounds and command both made its function more important than nostalgia. The buzz is at 60hz Tape or a frame tweak can shut one up sometimes,lol