Why does it have to be done and where do I get the validation?

You don't say what country you're talking about, but if it's France you have to stick your train ticket into the yellow composteur machine and get it stamped before you get on the train.

It depends what you are talking about. Usually that term is referring to railpasses, not single train tickets. It has to be done to indicate the official start of the ticket--railpasses usually have date limits, such as usuable within 30 days from the start date. There is probably also some checking of ID versus the owner (I had one once but don't recall all that), and maybe just scrutiny of the pass to make sure it isn't bogus.

As for tickets, someone might call composting them "validating them", I suppose, but I've never heard that. However, if that's what you mean, tickets are composted (stamped at a machine) to indicate that they are used, so they can't be turned in for a refund or re-used.

oh, if you do mean validating a railpass, you can get it done at any clerk at a window in the rail station.

It basically means you have to get the ticket stamped in a machine, generally before you board the train (in the case of buses, the stamping machine is generally on the bus itself).

This occurs when you buy a ticket which could be used on any train. By validating it, it means you won't be able to reuse the ticket for multiple journeys.

Validating machines are generally somewhere pretty obvious - like on the train platform. Just stick your ticket in, and the machine will automatically stamp it.

Hello eileen, aren't you going to Italy? If so you will see small yellow boxes at the train station. That is the box you use to vadidate your train ticket.