Originally Posted by AGHRMatt:
This poses one of those "just because we can doesn't mean we should" conundrums. Isn't there a restriction on the lengths of prototype trains running on US railroads? I know there was some controversy over a UP experiment with an ultra-long train.
I don't think that there is any actual legal limit on train length. But the longer it gets, the bigger the problems become in logistics. You have to consider siding lengths, passing loops, yard congestion and a host of other issues once you exceed a certain length.
But here is a link with some details of the UP experiments for anyone looking for a challenge. 
