What the heck, this was supposed to be posted to General not News/Activism. Sorry.
Put it in a paper bag and back on the train platform. You shouldn't have picked it up in the first place. Now you won't get any sleep. Or you could send it to Modok.
I think it's important how much you consider a wad to be...but if it is substantial, this is what I would do....create a new web address through yahoo or google or whatever. JUST FOR THIS, so you can cancel it after. I would put an ad in the paper...lost and found...just say money found on train platform...email this address:________________ with amount and which train platform and I will contact you if I did find your money. I'd run it twice and then if no correct responses, hey, it's yours...finders keepers. You don't have to donate it, ya know....