Well, I guess everything is a lot cheaper than a high speed line from London to northern England, but what value for money will 15/16 provide?
We'd only use the additional capacity to fit more short trains through Castlefield, e.g. the proposed Bradford - Manchester Airport service - since we are addicted to trying to provide a direct link from everywhere to everywhere else, meaning a mess of hourly services that try to cram through the Castlefield corridor.
A couple of extra platforms at Piccadilly will certainly improve things a bit (assuming we don't then try to squeeze more services through it), but there'd still be all of the flat junctions at each end (e.g. a Cleethorpes service trying to get onto the Airport branch whilst a service from Liverpool/Blackpool/Southport tries to get over to the Stockport lines)
And then, if you're going to have more Airport services then you'll need to add in the cost of additional platform space at the Airport (since there isn't enough at the moment to permit everything to lay over, hence TPE dropping three carriages of the Cleethorpes service at Piccadilly) - where will it end?
Don't get me wrong, it'd be better to do it than not do it, but hundreds of millions of pounds is a lot of money to spend when there are other ways of improving reliability (e.g. removing a couple of direct services through Castlefield each hour, like splitting the Liverpool - Crewe service)
I would agree the extra capacity shouldn't be used to cram tons more trains through Castlefield. However, splitting up current services may not be the silver bullet it is said to be. Firstly, capacity is limited for terminating services at Piccadilly, Oxford Road and Victoria. If you split the Liverpool Crewe service, you need to have one train terminate at Victoria, then another terminate at Piccadilly, that's extra space both those stations lack. Then, for all those passengers making the full trip, or changing onto a Piccadilly specific service (say Hope Valley, Sheffield, Buxton, Crewe, etc), they'll have to spend an extra 20-30 mins (& 2 quid) on Metrolink to complete their journey. Now some of the issues come down to TPE trains stopping thrice in Manchester due to Ordsall Chord. Really, Castlefield capacity should have been expanded before this was built, but we're here now and it's not going away. Spending the cash on Castlefield will be worth it, just to make good use of our existing infrastructure.
There is a trend of trains becoming longer, but then you hit the limits of the amount of people that can be crammed on the platforms at Piccadilly/Oxford Rd. Since the trains at Piccadilly/Oxford Road dump and reload a good 50-60% of their passengers at both stations, the physical space for passengers is reaching its limit. Things like accessibility are taking a hit, I've seen wheelchair users get stuck on trains, lifts are far too busy for those with heavy luggage & limited mobility. Trains can't even be scheduled to Platform 1 at Oxford Road due to the lack of a lift.
I'd argue, with 50,000 people a day passing through Piccadilly and far more on services through Castlefield, the fact Oxford Road has the most delays/cancellations in the entire country mostly due to the fabric of the station itself and passenger numbers that (if not this year), are likely to grow again in the future, it would be a very sensible spend. Plus, Greater Manchester itself continues to grow. It seems ridiculous that an area now quite overshadowed by skyscrapers and cranes, the rail corridor serving it is so outdated and under-invested in.
