A G-scale 1:29 scale EOT. Sometimes referred to as a flashing rear-end device (FRED) or sense and braking unit (SBU).

You will need:

1. a 5mm flashing red LED. I recommend one from Evans designs for a more prototypical flash rate. https://www.modeltrainsoftware.com/products/slow-flash-led?variant=28802793734192
2. a bit of 17Ga wire (bent to form the handle.
3. a JST PH2.0 connector. like this https://www.amazon.com/daier-Micro-2-Pin-Connector-Female/dp/B01DUC1O68
4. Battery and/or switch.

This was designed to fit a Kadee G scale coupler. Best to glue permanently to one.