With the tight packaging now being used by both Hornby and Bachmann, I have found the following procedure will help.
Before removing the loco from the packaging make a note of the areas that are tight and problematic.
Remove the loco and then with your finger carefully compress the packaging in the appropriate places, and then check the loco back in packaging.
This has eliminated any problems that I had, and hopefully will be of use to other people.
Places that I normally compress are as follows.
1. Wheel and bogey indents.
2. Buffer beam area.
3. Top of cab.
4. Tender wheels.
So generally this will solve problems, but carefull compressing the whole area will prove dividends.