Here are some real life Union Pacific steam punk Locomotives, sadly not in trainz
UP 4407
https://donstrack.smugmug.com/UtahRails/Union-Pacific/UP-Steam-DDR/i-H4rD2mx/A
https://donstrack.smugmug.com/UtahRails/Union-Pacific/UP-Steam-DDR/i-xTFZ5Vw/A
UP 768
https://donstrack.smugmug.com/UtahRails/Union-Pacific/UP-Steam-Locomotives/i-r6P6rLf/A
These strike me as somewhat steam punk perhaps with a bit grittier paint job. The one on the left is a steam engine BTW
<kuid:633834:100337> UP M10000 Streamliner
<kuid:633834:100338> NH Comet Streamliner
<kuid:633834:100339> CB&Q Pioneer Zephyr Streamliner
<kuid:633834:100340> CB&Q Pioneer Zephyr Mars Streamliner
<kuid:633834:100341> NH Besler Steam Streamliner
These are very steam punk. Especially without the tender.
<kuid2:37318:3:1> Reading 4-4-0 Camelback
<kuid:37318:7> Reading CamelBack Tender
<kuid:103021:100617> Reading B8a 0-6-0 Camelback Tender
<kuid:103021:100626> Reading B8a 0-6-0 Camelback
<kuid:103021:100800> Reading B8a 0-6-0 Camelback Undec
<kuid:103021:100801> Reading B8a 0-6-0 Camelback Tender Undec
<kuid:323193:100011> Reading Camelback
You might want to look at the Industrial Plant series done by kenichiro (seen in the background of the above screenie) and <kuid:-25:468> Building Factory has always struck me as somewhat steampunk.
edh6 and clam1952 did some Fairlies that are distinctive as are some Bogies done by trainboi1 and togog.
Good luck with your project!