The speed a normal rail curve allows can be calculated using the following formula:

Maximum speed in km/h = 231 - (13-CL)^2

Monorail and maglev use the same formula but get a 50% and 100% speed bonus respectively, resulting curve speeds are shown in the graph below:

For some more detailed information see Mark's blog post: http://www.openttdcoop.org/blog/2009/05/13/about-curve-lengths/