This article is from the Bicycles FAQ, by Mike Iglesias with numerous contributions by others.
I would avoid checking baggage(this includes bikes) on a train unless you
can afford to be separated from it for up to a week after you reach your
destination. This is because on European trains, the baggage cars are not
necessarily hooked up to the passenger trains which means you can wait for
days for your luggage to arrive(I saw this happen to a Canadian guy in 1990).
I have been told that there are some trains in Italy that include a special
baggage car the will hold bikes. You may want to check into this if your
proposed itinerary includes travel by train. The key here is that you want
to make sure that both you and your bike are on the same train.
On Swiss and German trains there is space at the end of most cars where
you can leave baggage, which is where I put my bike. In France and Italy,
I suspended the bike above the seats in the two opposing luggage racks(great
trick!). There is a chance that a conductor may be displeased by the bike
and start making all sorts of gyrations about the bike having to be sent
as baggage. Just play dumb and as long as you are not taking up too much
space they will usually let you slide.
Unfortunately, in 1992, I came under the wrath of every train conductor in
Europe. I never got separated from my bike, but I had to pay an extra charge
for having my bike with me on the train($30 US). However, I would rather be
verbally abused than be separated from my bike!
On interesting thing about bikes on passenger trains, in 1992 I took the
TGV from Paris to Pau and was not hassled about my bike because it was
still in the box and in the back of the car. You may be able to cut
down on your hassle quotient by keeping you bike in your box until you
really need it. Just a thought.
Train service is not available in all towns (especially in the mountains).
However, bus service usually is available and you can use the bus to connect
to a train station. Your bike has to travel in the baggage compartment,
it is a bit risky since the bike may move around a bit with all the luggage
so take care in helping the driver put it in a good location.