Train Station Lockers

Yes, they are often large enough to hold 2 or 3 carry-on suitcases. There are usually several sizes to choose from costing more money the larger the locker. Frankfurt for example, has lockers costing between 3-6 euro. You will need coins.

I've always managed to fit two regulation size carryon bags (not stuffed) into a smaller locker.

Here is an example of typical lockers (they're pretty deep). These are in Füssen.

Steamer trunks excluded, I haven't yet seen a piece of modern luggage that won't fit in the biggest rail lockers.

I'd guess, as an average, a small locker is big enough for two carry-ons, plus some small handheld items. A large locker is big enough for two larger suitcases, plus a few extras.