RD402 - Question

I saw RD402 on a work train on the shuttle in Times Square a couple of days ago. I got a close look at it since it on the platform. The interior still has advertisements from when it ran in service as #8956. It still has a nice shiny yellow/black exterior, but I did notice something weird near the center door. There is a fuel cap recessed into the side of the body, around floor level (of the platform), and it is stenciled 'Diesel Fuel Only'. Does anyone have any idea what this would be for?

Perhaps it is fuel to run a generator when there is no third rail power available? You (or anyone) wouldn't happen to have pics, would you?

nycsubway.org reports that R33ML #8956 was converted to RD402 in Coney Island Shops. Earlier cars with the RD designation were de-icers.
The car next to 8956, with its number already changed to 401, was R33WF #8986.

http://www.nycsubway.org/perl/show?30045