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

Maximum speed in km/h = 231 - (13-CL)^2
CL in the formula is limited to a maximum of 13.

For two curves in the same direction, CL is the number of half-tiles between the 45°-turns. If three or more curves are within the length of a train, the game calculates an average curve length based on the part of the train which is between those curves and the total number of total curves - 1. This can lead to strange effects: A third bend can increase the average curve length and allow a higher speed of the train.

Two 45°-turns with a single diagonal half-tile in between will always limit the speed to 88km/h, independent of other curves.

An example showing which corners are affected

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. Tilt increases the maximal speed by 20%.

See Also

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