TCS KA3, Keep Alive:
 
TCS Keep-Alive® (KA) devices are used to supply power to decoders during times of power interruption due to dirty track or problematic track work. The KA3 will power an HO-Scale locomotive with headlights on for approximately 6 to 20 seconds (depending on the locomotive and the efficiency of the motor).
 
This Keep-Alive® includes a Blue and a Black w/ White Stripe wire to attach it to a decoder.
 
For more information on how to connect Keep-Alive® including with other manufacturer's decoder products, click the tab above labeled "Documents."
 
MSRP is $31.95. (Picture courtesy of TCS.)