To add to what Paul says, look for Bruce Chubb’s How to Operate Your Model Railroad (Kalmbach, 1977, ISBN-10: 0890245282, ISBN-13: 978-0890245286) My local library has one. He has a chart listing basic percentages for cars, and Paul is right on for the '50s. Chubb also mentions other, less common cars and gives breakdowns depending on the road and what they hauled.
Well technically on the NP/Soo ore would be going to Allouez Wisconsin, on the far side of Superior WI. Only the DM&IR had ore docks in Duluth, all the other railroad’s docks were on the Wisconsin side of the St.Louis river/bay. [:)] BTW the NP did use mallets for a year or two on the ore lines in the early fifties, but decided to send them back west and primarily used 2-8-2’s on ore trains, often with a helper for part of the run. The “Mainstreeter” had a great article a year or two ago from a former NP fireman describing a typical ore run of the fifties from the fireman’s point of view. An expanded version of it is available on the NPRHA website.
http://research.nprha.org/Miscelanous%20Documents/Riding%20Cuyuna%20Iron%20Range%20Ore%20Service.mht
For the St.Paul part, except for the roundhouse in the middle of the wye by SPUD, most engine facilities weren’t too close by. You might be able to set up a couple of staging tracks to shuttle engines in and out to / from the layout. One leg of the wye starts to the east then makes a 180 degree loop around downtown St.Paul to head west towards downtown Minneapolis, so you could use that as staging perhaps. NP and CB&Q trains would use the Stone Arch bridge in Minneapolis to cross the river to the GN depot on the west bank in downtown Mpls.
Keep in mind that in the area you’re modelling, the Empire Builder and North Coast Limited would basically just be on the Burlington. The only NP passenger train from Mpls/St.Paul to Duluth/Superior was a local train which by the 50’s was often just a 4-6-2 or one GP or FP with 2-3 passenger cars. The NP did operate a 1-2 car passenger train (or later an RDC IIRC) from Duluth/Superior as a connection to the NCL route at Brainerd MN.