The speed record of conventional-wheeled rail vehicles is 574,8 km/h
By Alain Stoll - IMGP0492, CC BY-SA 2.0, Link
The speed record was made on April 3 in 2007 before the LGV Est line was opened for traffic on the 10th of June that year. The train in the picture is V150, a modified TGV high speed train. The normal regular top speed is about 320 km/h.
Here's a video of the successful speed record attempt: