Rich,
The original car that I am replicating still exists. It is in the compound area of the NSRM BC. Everything above the window drip rail is the original poplar that was put on the car way back when. It has stood the test of time for over 130 years. It doesn't look fantastic, but at least it is still there. Below the windows, they replaced the board and batten panels back in the 1880s or 90s with regular t&g car siding.
Poplar may not have been my first choice for siding, but I found out what the Billmeyer & Smalls guys found out: You can get it in large size boards and it is comparatively less expensive. The B&S boys wanted to make a profit.....I don't want to end up broke. so we had something in common in that choice of wood.
Dan