Planning Interrail might seem pretty straightforward, but there is a lot of work behind traveling by train.
Depending on your ticket, you are not able to travel with all trains. And yes, there are hidden costs. For some trains you have to pay a reservation or a booking fee.
Here you can find the different types of available trains. Remember that if you book a night train (especially in a sleeping wagon), you will often times have to pay an additional charge. However, this charge is usually very little in comparison to what you would spend on a hostel. Also remember that if you take a night train that leaves after 7pm and arrives after 4am the following morning, only the second day counts as a travel day.
Often times, high-speed trains also require an additional payment.
Take a look at this page to find out about the different types of seats and sleeping wagons/sleepers.
The Interrail train timetable is a useful tool that shows you exactly which trains leave from where and arrive when at your destination. It also shows you how long they take and if they require an additional cost or if a reservation is needed.