Totally different bikes. All X0-n bikes had 26 inch tires (the earlier ones) and loads of clearance to accept fenders on 38mm tires. They had different frames. The X0-4 had double butted CroMo frame, higher models had triple butted Ishi frames. All of them (other than the X0-1 which was a different beast on itself with RX100 road components) had MTB components (Altus for the XO-4) with ridiculously big chainrings for MTB (the XO-4 had 50/40/28), but even the Bridgestone MTBs had big big chainrings. The RB-T was a touring bike. 700c x 26 tires, RX100 triple components. Those 26" tire Bridgestone hybrids were really something very unique (and pricey) at the time. An X0-1 would sell for $1200 or so (if you could find one) back then... that's a huge premium for a hybrid bike.