Stranded or solid, 20 gauge is not close to being adequate for O gauge track power.
i often sit back and smile at these conversations, and i won't offer any counter argument, but if you actually did some practical research on this, 20 ga. wire in open air (how most train layouts are wired), will easily pass 5-6 amps which is a lot more power than any of my trains draw. AWG tables are hugely conservative for maximum power transmission (i.e. usually calculated as bundled wires over km distances) and even pushed to its maximum capacity 20 ga solid copper wire will drop less than a volt for a 10-15 foot run.
i do tend to use 16g stranded just because it is what i have tons of in many colors while lowering the Vdrop to practically zero as long as i keep the runs inside the confines of even large rooms, but after a while i get pretty tired of hearing people claim that you will have major problems if you don't use 12 - 14 gauge wire for even the smallest layouts. 1st off, you're wasting your money and B, i've got news for you that your bottlenecks and major losses are not going to be the wire, but will be in the type and number of connections.
i'm done, so go ahead and trash my comments if you like.
it's still the truth.
cheers, sweetness & light...gary