When calculating reach you need to keep in mind that, although you may be able to physically reach to 39", can you really do any significant work at that length ? Will standing on a chair extend your reach ?
Another consideration would be that, if the table will be strong enough to hold your weight and there are small areas where you can put a knee on and you are physically capable of climbing up, then 39" may not be out of the question. You can also buy a top-side creeper which will increase your reach.
My guesstimate is that only 10%-20% of engines need O-72 to run on and most loco's fall into the O-48 to O-60 category or smaller. That's not to say that all engines don't look better on larger diameter curves, but if we're talking minimum curve, then O-60 should be enough for just about all but the biggest iron.
