This is the Digitrax PX112-6 Power Xtender For N Scale 6 Pin Sound Decoders - Speaker Not Included.

-Keeps locos and sound running during power interruptions caused by dirty or bad track.
-Hold up time varies based on decoder load and track conditions.
-Use with Digitrax Series 6 Decoders with 6 pin sound harnesses such as SDN136PS and SDXN136PS.

Specifications
Size: 1.575in