FL9 Truck question

I was recently looking at current pictures of CDOT/MN and Maine Eastern FL9ms and I noticed the first truck had 4 wheels and the back had 6. Were they built with this arrangement or did it get modified in the rebuilds? Was it a B-A1A or a B-C arangement? What is the purpose of this arrangement? Does it have to do with the thrid rail pickup?

The B-A1A arrangement was to carry the water tank for Steam generator for heat.
There was a 600 gallon tank inside the car body between the Steam generaor and the main engine.
The big watertank pushed the Axle load beyond what Park avenue viaduct could handle so the engine was ordered with the 5 axle arrangement.
It made for quite a rough ride however as the engine did not have equal front and rear suspention.

Asa further historical note, that B-A1A wheel arrangement was very uncommon among American diesels but not unique to the FL9s. Fairbanks-Morse CPA24-5 cab units as found on the New Haven (NH 790-799), Long Island, and New York Central also had a four axle truck in front and a six axle truck with the center axle unpowered in back, and for the same reason, to distribute weight. The NH's C-Liners were very short lived and were retired in the late 1950s when the FL9s arrived.

A number of railroads, particularly in the West, liked to use F-units instead of E-units for passenger service, but it wasn't possible to carry enough steam-generator water on them for a long trip, so EMD designed a lengthened version with an A1A rear truck: the original FL-9, which was never built. This is described in an article by Preston Cook in the July 2008 "Railfan and Railroad": strongly recommended, like all of Mr. Cook's historical articles!
When the New Haven needed units with both passenger equipment (including boiler water) and the extra equipment needed for third-rail operation, EMD used the five-axle FL-9 configuration they had already prepared for a long distance passenger locomotive. Thus all the FL-9 locomotives actually built (60 for the NYNH&H) were of the variant design.
I think the FL-9 carbody was also envisioned for a never-built "FG-9" locomotive with a free-piston gasifier and gas turbine power plant.
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Fwiw: the basic F3/F7/F9 is 50 feet (and a few inches) long. The FP-7/FP-9 (a 4-axle variant lengthened to carry extra boiler water) was 4 feet longer. The FL-9 is four feet longer again.

No third rail gear was located in rear or beyond the F unit configuration.
The Third rail gear other than a bigger fan used the standard F unit dynamic brake grid hatch for third rail operation.
Only 600 volt component located in rear was a electric piston compressor to keep air pressure up on third rail.
It was located on top of the boiler water tank.
On later rebuilts by MNCR the Electric compressor was placed under frame in front of fuel tank.