The Wheatland Express is an excursion train operating out of Cudworth, SK and I believe its been around since 2015

I'm wondering about the entire passenger fleet that serves this "obscure" excursion train due to the limited information that's provided and the official source doesn't help at all

Link to their website: https://wheatlandexpresstrain.ca/

According to the 2023 Canadian Trackside guide, they have two passenger cars:

Diner 2017 "Lake Erie", ex. AMTK 8398, exx. UP lunch counter-diner 4001, nee UP cafeteria-lounge 4001 built by ACF in 1955.

Safety Car 2018, nee UP coach 4585 built by ACF in 1954.

Although you only asked about the passenger cars, I thought I would add their two RDCs:

#6120, ex. Seminole Gulf Railroad 6120 2020, exx. Cape Cod Scenic Railroad 6120 1991, exxx. MBTA 6120 1990, nee B&M 6120 1976. Built by Budd in 1955

#6122, ex. Seminole Gulf Railroad 6122 2020, exx. Cape Cod Scenic Railroad 6122 1991, exxx. MBTA 6122 1990, nee B&M 6122 1976. Built by Budd in 1955.

Elizabeth

Here's a short video from the power as it runs around a six car train.

https://www.youtube.com/watch?v=oiAFZkIF5HE

In addition to the four cars already mentioned, the stainless Budd combine is the former C&O 1402, later Amtrak 1600, most recently Tom Payne's "Styx" (numbered TPHX 19480). It remained in storage at Warden, AB on the Alberta Prairie Railway after Payne's death until being trucked to Wheatland's line a couple years ago, there are photos of the move buried in their Facebook page. The Trackside Guide still has it listed as being at Warden.

I'm not sure about the history or identity of the baggage car at the opposite end of the train.

The baggage car is ex-Santa Fe 3430 (Budd 1937), having been sold to Wheatland Express in 2021.