Question:
Anybody know an average travel time from Rotterdam to Amsterdam ?

Answer: Travel time ranges between one hour exactly and 1:15, depending on whichtrain you get. There are several trains per hour for most of the day; thefastest ones are called "Intercity." They all cost the same: f22.50 (US$11)one-way, f38.75 round trip. Weekend round trips are just f23.25.

You can find this and much much more information on their web site at:

http://www.ns.nl

I just checked; they have an English-language version.