Most stuff will work fine at lower voltages, but there are exceptions all over the place, and some really crazy ones you would not expect.
There are inexpensive 5 amp boosters that can be had and will work up to reasonable voltages, recently I put a portable system together with a Tam Valley 5 amp booster and used a 20 volt laptop power supply, giving 18 volts to the rails, which is fine for most demonstrations.
I've gotten "bitten" before, and there are examples of mainline diesels that need the full 24 volts DCC on the rails to achieve 92 smph.... (it was a passenger train, and I do normally run it about 75 smph, but they DID hit 100 occasionally)
Anyway, I have most likely once again beaten the topic to death... sorry!
Greg