That’s the thing - they’re not! The bogies can pitch. So, if they did what you suggest, the whole train could be a zig-zag if viewed from the side.
Even worse, even if the bogies truly were fixed axles directly mounted on the bogie frame, and we had pitch articulation between them, if the centre module were overloaded (say, by too many passengers), this would happen:
This configuration for fixed-bogie* trams is very obvious when viewing a 3-module LRV moving along track with a vertical curve, like the bridge in Kansas City right by Union Station. The whole LRV looks like a ridgid log when moving across it.
*- a little bit of a misnomer, because, as I’ve explained, the bogies can pitch relative to the module, and even yaw just a little bit, to take up shock/jerk loads, if spiral curves are not used (ie, Toronto’s legacy network).
Even worse, even if the bogies truly were fixed axles directly mounted on the bogie frame, and we had pitch articulation between them, if the centre module were overloaded (say, by too many passengers), this would happen:
This configuration for fixed-bogie* trams is very obvious when viewing a 3-module LRV moving along track with a vertical curve, like the bridge in Kansas City right by Union Station. The whole LRV looks like a ridgid log when moving across it.
*- a little bit of a misnomer, because, as I’ve explained, the bogies can pitch relative to the module, and even yaw just a little bit, to take up shock/jerk loads, if spiral curves are not used (ie, Toronto’s legacy network).