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# Trains & wagons

Sun 17 Jun, 2007 08:32 am

A locomotive is in charge of exchanging the positions of wagon A and B, then coming back to its departure position. It can go forward or backward but it can pass only one time inside the tunnel.

Is there a solution?

What if you can pass the tunnel as many times you want?
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markr

1
Sun 17 Jun, 2007 11:40 am
I don't know if it can be done in one pass. Here's a solution with 2 passes through the tunnel:

L moves left to enter circle
L moves counterclockwise to 2 picking up B
L/B continue counterclockwise to 1 picking up A (passes through tunnel)
L/B/A continue counterclockwise entering straight
A is left at 3
L/B move left to enter circle
L/B move clockwise to 1
B is left at 1
L continues clockwise to intersection of circle and straight (passes through tunnel)
L moves right to 3 picking up A
L/A move left to enter circle
L/A move counterclockwise to 2
A is left at 2
L moves clockwise to intersection of circle and straight
L moves right to 3
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Tryagain

1
Sun 17 Jun, 2007 05:23 pm
L enters track then anti clockwise, pushes B (LB) through tunnel collects A (LBA) and leaves A at point 3 (LB) L reverses To leave B at Point 1.

L continues reversing (clockwise) through tunnel past junction and forward to point 3 collects A back out to circle then anti clockwise to leave A at point 2.

Returns to point 3. Mission completed.

Which if I am not mistaken is the way Mark suggested.

I will say it is impossible to do this with only one pass through the tunnel.
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