I agree wholeheartedly with recommending the Manchester Viewing Park. Since you said you'd go by train, it's worth knowing that a bus operates between the terminal and the viewing park. I'd suggest you check on days & times of operation - I think it is only at weekends and Bank holidays.
Have a look on
www.ringwayreports.co.uk
The only thing against it is that there is very little shelter if the weather turns bad.
Rhys.