The golden rule of course:
NEVER presume or assume.
If you're travelling from Chiang Mai to Bangkok and then Bangkok to Phuket (Surat Thani) then you'll either fly, take a bus or travel on an overnight sleeper train.
If you decide on an overnight sleeper train, your luggage will remain close to you throughout the duration, so no worries there.
'Large overhead locker' on an overnight sleeper train? No, that's just the place you or you're new found compadre will be sleeping tonight. Either way, just enjoy the ride and your new-found friendship..
