On my server when someone interacts with a minecart the person is not inside the minecart but rather outside. but you can not Left shift to exit it not sure if this is a sponge or a forge issue.
Flash4203
Edit
Downgraded forge to 1722 and still have the same bug, so it is not forge.
MineCarts work on sponge version spongeforge-1.8.9-1722-3.1.0-BETA-1111
with forge 1722
MineCarts work on sponge version spongeforge-1.8.9-1722-3.1.0-BETA-1111
with forge 1726
Last successful build to work with minecarts was
spongeforge-1.8.9-1722-3.1.0-BETA-1117
with forge 1726
1118 and above errors
its defo a sponge bug
I also noticed that mobs can cross minecart track which is not meant to happen.