Oh my, a passing bandwagon. Right! *shamefully climbs on*
In my honest opinion, Manchester does need to start doing something to increase capacity, as all modes on the three current bus, train and tram system are busting at the seams in peak hour, and services are running slower and slower due to congestion.
I would welcome an underground system, as would any town, but costs in this day and age make it utterly unnaffordable. Whereas the works cost is quite affordable, add on the consultants surveyors, mission-impact-survey-proffessionals et-al then the price goes through the roof.
The only affordable way forward for Manchester is the further expansion of Metrolink, further cross city tram routes, additional rail terminal stations, additional tracking (4-tracking) from Deansgate to Piccadilly, and bus-gates (bollard protected bus lanes) running from the city for 5 miles on the main bus corridors.
Leigh needs to be re-worked to be made as a Metrolink line, as the current busway project is pointless - it duplicates a free-running part of the East Lancs Road, then joins the East Lancs Road just where the traffic starts to build, ie straight into 8 miles of congested traffic jam. The line could either be routed via the Port Salford / Trafford Centre planned line or into north Eccles, then via a new line next to the M62 & the Eccles rail line. Although I would much prefer Leigh to have a proper rail line, there is no way we would get funding for that from TfGM... if it ain't Metrolink, they ain't interested.
New rail stations are needed at key congested road arteries to increase Park an Ride such as Leigh South (East Lancs Road).
'Busting at the seams' certainly applies to the trains (at least from Bolton and the north) and Metrolink. It is less true of the buses - which run full for a much shorter period.
Currently there are 140,000 jobs/student places in the city centre, including the Salford and Manchester education precincts. Some 30,000 workers/students live close enough to be able to walk/cycle daily. So the transport system has to handle around 110,000 peak period trips. Approx 45,000 of these are by car, 35,000 by bus, 23,000 by train and 7,000 by tram (all 2010 figures).
But the city centre is currently adding 3,000 to 5,000 jobs per year - so over the next ten years, it is likley that commuter flows could increase by around 40,000. As you say, there is not currently the capacity.
However:
- the Metrolink is expanding from 31 trams (in 2010) to 94 trams (in 2015). This may be expected to increase peak period tram ridership by around 14,000
- the likelihood is that three at least of the east-west current heavy rail commute lines will convert to tram-train operation. This should add a further 5,000 commuter capacity over each peak period.
- the Northern Hub will add two extra platforms at Piccadilly; which, together with another two platforms released there by the tram-train conversions and new platorms at Salford Central and Salford Crescent, will increase peak period capacity for heavy rail commuter services by around 7,000 per day.
- the Leigh Busway is a perfectly sound project; as it is linked into the creation of dedicated bus-lanes all along the East Lancs road into the centre of Salford, and then across the city centre to Manchester Uni. Together with the three 'cross-city' bus routes (to Didsbury, Middleton and Earlstown) , this will add a capacity of around 4,000 bus places in each peak period.
So the service expansions currently funded and underway should provide an extra peak period capacity for around 30,000 commuters into central Mancester by 2016.
But - if the jobs growth is more rapid than currently, or anyway once HS2 arrives on the scene and dumps an additional 8,000 peak travellers per hour into central Manchester - then still more capacity could be needed.
Possible further expansions:
- double all the Metrolink services. The currrently purchased 94 trams only allows one (out of four) Metrolink lines to be doubled. Buy another 50, and we could double the lot.
- create a Manchester Overground. Once the Ordsall Curve has been built, it will become possible for most north-south suburban commuter services to be doubled in length, and to run through the city centre (e.g. Hazel Grove to Bolton; Macclesfield to Rochdale) much on the same pattern as Thameslink. The key constraint is the pinch-point at Deasgate Station (Oxford Road already has four through platforms)m, which would need to be four-tracked. This won't be cheap; but will be a lot less expensive than going underground.
- extend the cross-city rapid bus system. The key need is for a service to run along the Stockport Road and through towards Bolton. Again, not cheap, but it would be a major addition to bus capacity.
Taking all those together, and there could be an extra capacity for 30,000peak period commuters by 2030.
Which, combined with further city centre residential development, should mean that Manchester will only begin to feel serioulsy squeezed for commuter capacity once City centre jobs top 220,000 (or 80,000 more than today).
It was really just to stir the pot a bit because Newcastle has an underground and Manchester doesnt 
