Essentially ( and simplistically ) Route Availability is a) does it fit? ie will whatever you're testing physically fit through bridges/tunnels/stations/other infrastructure ( while moving, too ), and b) will it destroy the infrastructure? ie will it weaken the track/bridges/other permanent way features or even just are they strong enough to cope. The last one is mostly axle loading which is a concentration of force in one spot ( which isn't always all pointing down either ), but in steam days you'd have to account for hammer blow too, although not sure if that really had an effect on where anything was cleared for.
The Westcountry pacifics were, as the name suggests intended to be used in the south-west - and yes, there's plenty of photos around of them hauling services on the branch lines down here with a load that's probably less than the locomotive itself. Not so much replacing tank engines ( the lines were lightly used but generally long & hilly - which is not to say there weren't any used ), but N1/U1s ( moguls ) and T9s, which dated from the turn of the century & lasted forever. Using a huge pacific in those cases was just as wasteful as you'd imagine.
The E1/R tanks in the south-west were rebuilt from a class originally from the 1870s, which makes the T9s look modern. You can see why the pacific idea came about.