I have just recovered the server from an issue where a simple spawn & transport elevator has created 1900 minecarts and crashed the server massively, and repeatedly. It took me almost one hour to fix this. Due to the massive amount of minecarts in the area, it was even hard to go there without my minecraft client freaking out.
I therefore made the decision to (as discussed earlier) remove minecart auto-spawning from the system. Therefore, unfortunately, all minecart riding systems must be now manually supplied with minecarts that you have to craft yourself.
As a compromise, I am thinking about making a command that would simply give you a minecart instead so you can place it without having to craft it. This will not happen in the next 6 hours however.