The maximum speed of a train in a straight line is 8 blocks per second.
If you place a powered rail every 12 blocks, you will travel at 8 blocks per second.
If you place a powered rail every 24 blocks, you will travel at 8 blocks per second.
If you place a powered rail every 37 blocks, you will travel at 8 blocks per second.
After that, you don't stay at max speed (one every 45 blocks is only 7.1 blocks per second, 50 is 6.5, etc).
(And if you want to argue, go argue with
this guy who actually
tested it.)
EDITED: 6 Aug 2011 17:42 by BOUGHTONP