R68s on the B

I take the B to work most mornings from CPW to 47-50. Ordinarily R40s exclusively run on the B (so far as I can see anyway), but recently I've seen trains of R68s (which usually run on the D). In the 8 months I've been taking the B to work this is the first time I've seen this-- does anyone know if there is any particular reason why?

I'm pretty sure I read somehwere that there is an odd r68 set or two on the B, and it wouldnt be unthinkable that you had simply never had the fortune to see it before.

On the other hand, they are supposedly phasing out the r40s in favor of the new r160s, and the only line I have seen the r160s on in regular service is the N.

They may be shifting r68 sets from the N onto other lines as more and more r160s are brought into service on the N.