If you meant to say "on the tracks", I would say it like "在站台下面" 或者 "在铁轨(铁路)上面"。"在火车下面" sounds frightening, 好像是在说 "被车碾过 (run over by a train)" 似的。
To me, "在桌子下面" would strictly mean "below the table". If something is on the floor, near/next to the table, I would say "在桌子旁边"。 Or, if I choose to be vague (because I'm frustrated or something), I might say something like "在桌子那儿!"。 Now, this would indicate that the item of interest could be anywhere on the table, beneath the table, or near the table.