The road specific locos are a small market for most. Most Class 1 railroads bought USRA standard Light Mike's and Pacific's during the late teens and twenties. I model the SAL and like the Q3 myself but they are the only RR that had this type. They were forced like everyone else to buy USRA types during that period. As mentioned,  the heavy pacific wasn't widely used with the exception of the  Erie, Southern and a few others.

Do you think you will ever offer a N Scale USRA 4-6-2 Light Pacific or USRA 2-8-2 Light Mikado? You would sell thousands. No one makes a decent one. The Model Power locomotives have cast in handrails. Seems like a no brainer.  As well as you have done with all of your other steam locos, I have many, I think it would definitely please the N Scale crowd.

If Bachmann offers the tender for sale by itself, the new EM-1 tender is a dead ringer.

Have you ever considered making a N scale 6 wheel truck Long Haul Tender like the one used behind the Bachmann HO 2-8-4 Berkshire? Most of the east coast 4-8-4's also used them. If you did you would have a close facsimile to what was used on the RF&P, Western Maryland, C&O Greenbrier, etc. I know the L&N 2-8-4's also had a similar tender as did the Southern PS-4 and various C&O 2-8-2's and 4-6-2's. Your off the shelf 4-8-4 would be a nice start linked to one of these. The N&W J-Class Tender is unique to that locomotive but similar in size.