Round circuit board on the end of Toby's motor

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I posted this question in the Thomas section and got no replies.

I am modifying a G scale Toby to run with a BlueRail board. Inside Toby, a set of orange and brown wires lead to a small round circuit board on the end of the motor. It seems to have some resistors and a few capacitors. What does this circuit board do? Can I leave this circuit board in place? (BlueRail uses the same pulse-width-modulation to run motors that a decoder does to run motors).