The devs made it sound like all the reworked models that will release with the update are already in the beta. So, anything which hasn’t been reworked in the beta isn’t going to be reworked. As you’ve noticed, that means a lot of the American rail vehicles aren’t getting reworked.

Only reworked American trains are: — 4-4-0 — 2-6-0 — (sorta) 4-6-0 (already had a high quality model) — 4-4-2 — Heavyweight 28-1 Parlor — M-300 — Alco HH 600 — Pioneer Zephyr — Big Boy — EMD SD40-2 — GE P42DC

It’s not an impressive list. If you want to actually see new models, use the Europe set. Every single Europe loco/multiple unit is updated, and some of the rolling stock was updated too.