So after some testing with 2 parallel bi-color leds with a 470R resister and an unloaded 12vdc can motor, here are some results. (The test motor is about 1-3/8" diam x 2" long and quite heavy. Its resistance is about 5.2 ohms. It's manufacturer is not known but it's not a Mabuchi or any other common train motor.):
2.2vdc: Red leds steady but dim; motor just turning over. Green leds behaved same but needed 2.8vdc.
4.0vdc: Both colors fairly bright, motor running faster.
6.0vdc: Both colors comfortably nice and bright.
8.0 and 10.0vdc: No change in brightness to speak of.
When the voltage is reduced both colors dim noticeably below 5vdc.
This is all highly subjective of course, but in conclusion it looks like voltage levels above 6 or 7vdc will produce acceptable brightness, which agrees pretty well with grj's assessment above. Next step will be some actual engine testing. Since this board is exactly the same in size etc as the class led board put in my Soo Line MP15 switcher a year or so back, that will be the easy to mod test bed! More to follow.......
