Thanks, David, Eddy, et. al...
Here is the full scenario. I have actually been pondering this for more than a year. I sometimes shoot trains at a certain east-west track wherein I am faced with something of a side-lit scenario--the sun in the southern sky with the track east-west. Thus, during a fair portion of the day, I am prone to getting some shadows on the locomotive's nose. If I could just get the flash to reach the loco's nose, the shot would look much better.
I have been using my 50/1.8 or 1.4 or 55/1.2 for this shot, but will soon be experimenting with my 200/4 as well. In any case, since a 50 is good for focus from about 30 feet to infinity at f/4, I had been thinking of using the 50 in this manner, as the nose of the loco tends to be about 30 feet away. I had originally mentioned 40-50 feet just to assess what my absolute maximum range might be. Overall, 30 feet at f/5.6 would provide a little bit of "insurance," so to speak. At this locale, the train moves fairly slowly--usually in the range of 15-25 mph--and so 1/250th should be sufficient on most days.
I am appreciative of the advice, as, with regard to fill flash shots, I have worked only with program mode (that was years ago with my N80 and some much closer subjects). Thus, when you say "1-2 stops below ambient," I gather you mean that if I am metering the scene at 250 @ f/4 I should stop down to f/5.6 or f/8 and take the shot? In this scenario, I gather I would be using the flash on manual and at full power? Also, how would I go about setting the flash? I will purchase either an SB16B or SB28. As long as the SB16's power is adequate, I would rather remain more "period correct" with the FE2--again, if possible.