Schwinn Meridian trike not pedaling
I'd guess the single-speed freewheel/freehub on the jackshaft has something wrong with its pawls.

That's the bit that lets you coast along without pedalling, and the rear chain will keep moving with the rear wheels.

To fix it temporarily you could try flooding the freehub with solvent and wiggle it lots.  When its moving better let the solvent dry off, and lubricate it after a while.

The permanent fix  would be to replace that freehub unit on the jackshaft.

The prevention would be to store the bike in a garage or inside, and to rinse the transmission off after every winter ride - I see a bit of corrosion there which could indicate the cause.