This may seem like a dumb question, but my curiosity was aroused when I watched live on a web page a seemingly endless train pass a crossing in Indiana. I was wondering if there is a "legal limit" to the allowed length of a train and if so is this a National limit or does it vary within States or geographical areas. I do understand obviously they cannot be as long if there are major elevation changes, weight restrictions and etc., but was curious if there is what could be called a legal length limit for trains operating over main lines in the U.S.

Thanks for any informed reply.
Regards,
Rolland Meyers