When I use a minecart with furnace on an unpowered rail system bot alone, pushing and pulling other mine carts it will randomly depower after taking a corner, that is act as if it has been overstressed and slow to as stop. this seems to have a random chance of occurring after any corn er but on average will take effect after 3 or 4. as you can imagine this renders unpowered rail systems almost useless especially due to the random nature that this bug seems to have.

Steps to Reproduce:
1. build a medium sized square of track approx. 6X6 blocks
2. place a furnace mine cart on the track and activate in any direction
3. Observe the cart depower after approximately 3 corners