Most of lionel stuff is AC.
Most AC items are ok on dc power. It's the dc items- boards and can motors that will burn up on AC power.
An AC safe board changes AC to DC before it does anything. Then the DC is used for logic/chips and or driving can motors. The boards use 4 diodes or an 4 terminal equivalent called a bridge rectifier to coral the two ac line's +and- pulses into flowing one direction only from + / - terminals (pulsed +/- current, but real close to smooth dc, motors and lamps, etc can't tell. A capacitor smooths the pulses even more for more sensitive uses )
Look up a diagram of a bridge rectifier. ( a diode is just a one direction gate, only letting flow happen in one direction. A BR forms a diode corral that runs power in a SiMPLE circle/trap/maze of sorts, the ac is free to add or take + or - as needed and the opposing terminal is always in an opposite state. Positioning of two more gates form new loop of travel akin to a one way cow chute that is fed when either ac line pushes, then dumps back into the corral where the neg ac wave extracts when needed.
