The maximum speed for rolling stock function is a built-in feature for TS2010.
Add;
extensions
{
max_permitted_speed-60850 "##kph"
}
to the config.txt file of an item of rolling stock (replace the '##' with the preferred speed and you can use mph if you like) and the item will be speed limited up to the speed stated in the extensions container.
Where an item of rolling stock already has an extensions container you simply add ' max_permitted_speed-60850 "##kph" ' to whatever is already in the container.
This function also works for locomotives.
For your information, the '-60850' refers to the user id of the original creator of the rule in question.