ATSF F unit Numbering

Yep, L = Lead.

My guess is it wound up being a carryover from Santa fe's first EMC boxcabs, the 1 and 1A, where 1A was the "second" unit of the pair and rather than going xxxABCD the Santa Fe just continued with xxxLABC. 

Notice "L" never appeared in the numberboards, just as a very small letter on the nose above the anti-climber. 

Rusty

You're correct.  But once you know this and take a second look at Santa Fe cab abd booster diesels, it becomes more complicated.

Santa Fe numbered the controlling unit of a cab and booster locomotive with an L-suffix to denote lead unit.  On a 4-unit EMD locomotive, the cab units had L and C suffixes, while the booster units had A and B suffixes.  The L never did appear in the illuminated number boards, and the C did not until the 1950's.  These locomotives were kept together in numeric sets until the mid-1950's, when one unit might go to the shop independently and be temporarily supplanted by another unit.  Therefore there could not be a 35L and a 35C - each only showing 35 in their number boards - on different trains causing possible safety issues with train orders if they happened to meet somewhere.  That unlikely safety issue was removed by painting the C suffix in the C cab unit number boxes in the early to mid-1950's.  At the same time, a small letter L was painted in black paint at the bottom of the yellow nose stripe, just above the anticlimber, to identify the L unit.  On train orders the 226L or C, whichever way the locomotive was pointed, would be identified as "Engine 226" until the addition of the C in number boards, after which the 226L would be "Engine 226" and the 226C would be "Engine 226C". 

Although we see that, in the east, Pennsy obviously marched to its own drum in just about every way.  Santa Fe, in the west, was inclined to do things its own way, at variance with more common practices adopted by most other railroads.  You just have to look deeper to find it, because the railroad and its equipment did not look as different as the Pennsy did.  You have only scratched the surface with the L designation for lead cab unit numbering.  But, to completely answer your question:

• The reason for the numbered units remaining together for over a decade was rooted in union contracts where the BLF&E maintained that every diesel unit was a locomotive and should have a Fireman and the ATSF maintained that every locomotive consist was a locomotive and one Fireman would be used on the locomotive consist.
• The FT's were renumbered and reconfigured into bobtails and out of bobtails so drastically (over the BLF&E issue, and - in some cases - to fit them for local service before the GP7's arrived) that you should regard them as a book-length study of their own.  Nobody - literally, nobody - is willing to authoritatively state that all the number changes are recorded.
• When  the Alco PA1 and PB1 locomotives 51 through 62 were delivered as 3-unit A-B-A locomotives, the cab units were numbered with L and B suffixes, and the booster had an A suffix.  Again, the B did not appear in the number board.  The railroad needed more flexibility so that 3 units could pull the Chief, and two could pull the Texas Chief, so the B-suffix Alco cab units got got new numbers with an L suffix, beginning with 63.  After the rebuilding of the original 3-unit 51 by EMD, the 63L became the 51C, and did display the C in the number board.
• The Erie-Built, Number 90L-90A-90B was the only cab-booster-cab locomotive of its type on Santa Fe, and the 90B was never renumbered, and I suspect - but cannot verify - that the B was painted into the number box in the '50's.
• 300-Class bobtail passenger units and 325 Class dual service rednoses, delivered as A-B-B, were suffixed L-A-B, and a small number of 2-unit A-B 300s were suffixed L and A.

Yes - L indicates lead.

Thanks.

I knew the letters were used to have a multi-unit set as one locomotive for union issues.