TCS KA2, 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. 
 
Newly-redesigned at half the size of a KA4, the KA2 will power HO or N scale locomotives with approximately one-third of the capacitance of KA3/KA4 devices (depending on the locomotive and motor).
 
This Keep-Alive® includes a blue and a black w/ white stripe wire with which 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.)