This is effectively a 200mF keep alive circuit I installed in a small 2-6-0 steam locomotive.

Project cost: $~3.50

Here's the physical construction

Load testing before shrink wrap is applied

Soundtraxx Decoder Interconnect Details

Soundtraxx decoders use a JST-SR 2 pin header

Receptacle- https://www.digikey.com/product-detail/en/jst-sales-america-inc/SM02B-SRSS-TB-LF-SN/455-1802-1-ND/926873

Harness Plug- https://www.digikey.com/product-detail/en/jst-sales-america-inc/02SR-3S/455-2183-ND/1300342

12" harness https://www.digikey.com/product-detail/en/jst-sales-america-inc/A02SR02SR30K305B/455-3017-ND/6009393 (can be cut in half to make to keep alive connectors)

Crimping tool is over $1000, so I won't be doing these by hand anytime soon.