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The best way to fix this problem is to gently insert a hobby knife blade first in between the prongs and GENTLY pry them apart. Being careful not to cut yourself with the razor edge. Do this for all four parts of the connection until they look evenly spread but not so much that you can't insert them into the connections at the battery car or motor. This will solve the non-working condition 90% of the time.
The 7722 also lends itself to being converted to 9v very easily. Simply remove the 4.5v motor and build the 9v motor into the frame of the 7722 engine. I did this several years ago and it looks good and works great.