Was wondering if i could use a left over Tsunami Capacitor
for constant lighting in a passenger car
When i installed a Current Keeper to the Decoder i cut off the smaller capacitor as instructed
Hate to just throw it away
If so how do i wire it ?

Probably if you have more than one. 250ufd is not much capacitance.
Original DIY keep alive using capacitors like this was much more the 250 ufd.
Rich
I wonder if by it’s self it would atleast jeep the lights from flickering
It might help a bit, if the lights are LEDs and have resistors chosen so that they are drawing very low current - 1K or better for each, which would put them under 10ma each.
Or, it will work as a filter capacitor to get smooth DC from the track power - connect the AC side of a bridge rectifier to the track pickups, put the cap across the DC terminals (using correct polarity, unless you want to emulate the Lionel exploding boxcar). With a low current load it will be enough to smooth the edges of the rectified DCC.
–Randy