Gaugemaster GM52 Multi Scale Track Tester.
This track tester works on 00, H0 and N Gauge track and enables you to quickly test track for DCC or DC power as well as also helping you to locate faults at track joints, points etc.
To use, simply place it across your track and the multi-coloured LED will either display Orange for DCC power, or Red and Green for DC power with the colour indicating the positive or negative. The polarity (or train direction) of the rails is shown on the product.
The tester measures 23mm (length) by 18mm (width) by 13mm (height).