Dany 
This is the most obscure, and also cheapest, souvenir I've ever bought: a Bahnsteigkarte, bought on the last day possible, from the last public transport company in Germany to sell them.
smekiwandert@Dany but its usual now that you have to pay for a ride to enter subway platforms only to pick up a person. Also the ÖBB here in Austria advises you to keep your ticket until you leave the platform of the railway station. So "Bahnsteigkarten" are still common but in another form.