Hello & Welcome to the Forum!

I've discussed your problem with a friend, a retired Lionel Repairman.  If it you have the capability to drill a hole into the center of the broken bolt you can take a fine/very small easy out and remove the remaining piece.
If that is not possible, your only option is to remove the wheel/blind driver and replace it and the bolt, being careful to get the wheel in quarter when replacing it on the axle.

I wish there were an easier answer for you, but replacing the wheel is probably your best bet.

Happy Rails!
Dave