Noel there is a trend on systems with double-ended vehicles to prefer to have validators at the stops in order to avoid door congestion caused by people slowing down to swipe at doors.

With single ended vehicles (doors on one side) it's possible to address this problem by putting validators on the opposite side of the aisle to draw people away from the doors. When people are boarding and alighting either side, there's no practical "opposite side" on which to install scanners.

About the only thing you can do in a double ended vehicle is fit the scanners facing the aisle and away from the doors so that people aren't actually in the doorways when they scan, but it may not draw them far enough away. Dwell time 101, subject strand 3b ;)

Tony P