Now, I could be saying something stupid here.  But I think we're missing a definition of what happens at the topmost boundary.
Say our number is 999,999, if you subtract one, you don't get a lucky number, but if you add one, then what happens?
The problem said that it was going to be a 6 digit number at all times, but if you add one, it becomes a seven digit number, so does this case never occur?  And if it does, then does the lucky number formula apply only to the last 6 digits of the number?

You are right. In fact all the tram tickets have exactly 6 digits, so ticket 1000000 doesn't exist. I have added this phrase to the hint.

It doesn't really matter, because the problem definition says the sum of the first 3 and last 3 digits differs by one.  999,998 fits this definition, but 999,999 does not.