A transmission with interlock mechanism which prevents an inappropriate shift

A transmission comprises first and second shift rails 12, 112 which actuate synchronizers (16, 116, fig 1) and are electro hydraulically operated via pistons (18, 120). A mechanical interlock mechanism having detent ball 32d which is arranged to be received in a recess 133 when the shift rail 12 is moved from neutral to engaged a gear via synchronizers (16, 116). The shift rail 12 pushes, via balls 32a-32c, the detent ball 32d firmly into recess 133 so that shift rail 112 cannot move until shift rail 12 is back in neutral. Detent ball 37, 137 engage in recesses 34a-34c, 134a-134c when shift rails 12, 112 are moved to select a gear or neutral. Non-contact position sensor (40, 140) having ferrous activators (42, 142) are provided to monitor the position of the shift rails relative to a neutral position. In case of failure of an ECU, the interlock prevents damage since it will not allow engagement of two gears at the same time.