Similarly, in the UK train axles must not be more than a set distance apart, otherwise the train could disappear (from the signalling system). This is one of the reasons we can't have double decker trains, as the carriages aren't long enough.
For the curious: double-decker carriages do not have to be shorter or have shorter axle distance, per se, but in the UK, they have to because their rail network is older, and can't handle the load that a double-decker train of 'normal' length exerts on the rails. https://en.m.wikipedia.org/wiki/Bilevel_rail_car:
"Bilevel cars may not be usable in countries or older railway systems with low loading gauges. This includes much of the rail network in the northeast of the USA and almost the entire British rail network. In some countries such as the UK new lines are built to a higher than the existing structure gauge to allow the use of double-deck trains in future."
It's not the load: the loading gauge refers to the maximum dimensions of a train. In the UK, the vast majority of lines bridges and tunnels are simply far too low to allow for double-decker coaches; the only ones ever used had pretty weird offset compartments: see <http://dart75.tripod.com/cutaway.gif> for a cutaway diagram.
