I had a wonderful time on my solo trip using the single rider lines in November.
Disneyland:
Splash Mt. - ask the CM collecting fast passes for singel rider pass, enter thru exit. Wait for CM to direct you over the little bridge (or they may even direct you to a seat right away!).
DCA:
Mulhulland Madness - seperate marked entrance, just enter the gate.
Soaring- ask fast pass CM if any single rider passes available (they do run out)
Screaming- ask fastpass CM for pass, enter thru handicap entrance. Take elevator up walk to next elevator and go back down.. then just wait for a seat.
I read that Mallaboomer occasionally has single rider but did not see marked entrance and didn't want to ride it any way
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Does GRR have single rider??? (Only during busy times???)
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Some other tips for fastpass..
Some rides are "unplugged" from the system.. meaning you can get a fastpass even if you are already holding one ..
The unplugged ones are:
Roger Rabbit Cartoon Spin
Star Tours (when open for fastpasses- they offer fast pass for this ride only on weekends or busy times)
Haunted Mansion Holiday version
GRR
The following ride/attractions no longer have fastpass:
It's Tough to be a bug
Muppet Vision
Pirates
Pooh
Of course all this is YMMV!!