#2             87. TRICKY TRUCKS             
Ernie the engine driver looked around in despair.

“It’s taken me all the morning, and now I’ve got it back to front,” he groaned.

“What’s the problem?” I asked.

“Truck C should be at warehouse B and truck D should be at warehouse A,” he explained. “The problem is that while the trucks will fit under the bridge, the engine won’t.”

“Well, that shouldn’t be too hard,” I mused thoughtfully.

What do you think? Can you interchange the positions of the trucks and return the engine to its present place? The engine and trucks can be coupled together, of course.
HINT 1

Use pencil and paper, or a little model, to play with the problem. (All good problems deserve to be played with!)
HINT 2

Try approaching the problem logically. Can you put truck C in the wanted position? Where would you go from here?
SOLUTION

Push C under the bridge, travel anticlockwise to push D, and couple C and D. Take the trucks to East Line, push C back onto the main line, and then push D back under the bridge. Now push C to West Line, and pull D to warehouse A. Proceed to couple with C on West Line, drag C to East Line, and push it in front of warehouse B. Finally put the engine back in place.

EXTENSIONS

1. There is no end of possibilities here! Variations in track layout, numbers of engines and trucks, possible obstacles ... . Remember that simplicity is desirable for a good problem.

2. You might try constructing a problem by working backwards. Start with a desired sinple final position, and see if you can get a difficult initial position.

3. Try working with another person, each devising a challenging problem for the other to solve.