Hopefully I can add a little to what The Johnster has already told you. The North and West route expresses & freight trains all changed locos at Shrewsbury prior to the arrival of the warships & westerns when the changes were made at Crewe. Counties did not make it to Crewe, I believe due to their overall cylinder spacing being out of gauge with EX LMS platforms. It should also be said that any western engines that did make it on freight trains would use the station avoiding lines and change engines at Gresty Green.
Parcels trains on this route could turn up with anything at the head.
Prior to dieselisation Local Passenger trains always seemed to be formed of EX LMS stock. The mainstay motive power was Stanier tanks but anything, particularly ex works could turn up so it was not uncommon to see a Duchess on 3 woebegone none corridor coaches. These trains used Platform 4B, now 7.
Whilst on the Shrewsbury route it is worth mentioning that at Whitchurch there was a direct connection to the Cambrian lines via Oswestry. The through service from Aberystwyth to Crewe almost invariably brought a Manor into Crewe. Some of the Oswestry - Whitchurch also ran through to Crewe, again almost always with Manors. One of these trains also conveyed milk tanks as tail traffic. These trains usually terminated in Platform 1B or 2B neither of which exist today as they are part of the extension not to be modelled.
Crewe - Wellington passenger services were provided by 2 B sets as the norm. Normally hauled by 4158 or 4178. Around the late fifties, BR 3MT tanks were regular for a period, not sure of exact dates 82005 & 82009 were regulars followed by Ivatt 2MT tanks 41232 & 41241 being regulars. Loco substitutions were usually Wellington based 8750 tanks though a Hall has been known to have been used along with Collett, Hawksworth & MK 1's . These trains used Platform 3B, now 8.
There were no through passenger services over this line Except for The Pines Express for a couple of summers in the early sixties, almost always an EE Type 4 diesel.
Parcels trains on this route frequently brought castles into Crewe.
Freight traffic was headed mostly by western steam and almost anything could be seem but again would not been seen in Crewe station as engine changes were again done at Gresty Green.
Hope this helps and look forward to your future progress with this project.