The thing that finally made it clear to me was an example explaining how は defines the topic at the exclusion of other things.

9時に東京から大阪に電車で行きます - I go from Tokyo to Osaka at 9 by train.

9時に東京から大阪に電車では行きません - I do not go from Tokyo to Osaka at 9 by train - but by some other transport.

9時に東京から大阪には電車で行きません - I do not go from Tokyo to Osaka at 9 by train - but to some other destination.

9時に東京からは大阪に電車で行きません - I do not go from Tokyo to Osaka at 9 by train - but from some other origin.

9時には東京から大阪に電車で行きません - I do not go from Tokyo to Osaka at 9 by train - but at some other time.