The existing rail service from Castleton into central Manchester is relatively infrequent - only 2 tph - and travelling from Heywood to central Manchester via Castleton is indirect.
Indirect? Not particularly. It would only be 5 mins from Heywood to Castleton and then it's around 17 mins Castleton to Manchester Victoria.
Yes, you'd have to provide a decent connection but the Calder Valley line isn't that busy to make that impossible and keep the Heywood to Victoria journey time below 30 mins.
Heywood to Victoria by car can be as bad as 28 mins in the peak, and then you have to find extortionately expensive parking. The nearest Metrolink park and ride is Whitefield (approx a 15 drive away), and that's busy in the mornings too. Add in the Metrolink journey time into central Manchester and it's not competitive with a Bury via Heywood shuttle, connecting into Victoria bound services at Castleton.
Using the existing rail line from Heywood to Bury will cause conflict with the preserved East Lancashire Railway and doesn't provide a direct link to Bury Interchange.
The ELR will have to compromise at some point, a lot of those north GM towns have some of the worst deprivation issues in the region and reliable public transport based access to well paid jobs in central Manchester is crucial to try and tackle it.
Bury Bolton St is a 5 min walk away from the interchange, it's hardly Timbuktu.
In general, Metrolink is too expensive to provide solutions for orbital public transport corridors in Greater Manchester, e.g. from Rochdale to Bury, or for that matter from East Didsbury to Stockport. There does not seem to be an economic or transport justification for this latter recently funded extension, merely a political one.
I wasn't suggesting Metrolink initially personally, a DMU or battery EMU shuttle would do. If they can get funding/permission for Metrolink, great - it's an area with relatively high population density, a fair few particularly deprived areas and very poor connectivity since some particularly poorly advised closures (especially the line to Accrington via Haslingden - the line today would have easily 75k+ in catchment without a rail service in reasonable proximity now and plenty of commuting into Manchester from large villages in the area like Helmshore and towns like Ramsbottom).
Have you ever envisaged the amount of HGV, let alone private motor vehicles, that daily pass through the Simister Island which is a major traffic hub on a cross-Pennine major motorway route.
You cannot make a numerical comparison with Simister Island and a minor rail line.
I agree there's a lot of strategic traffic. The reason for the constant congestion is the amount of commuter traffic that is forced onto the M60 as the only suitable option for their journeys, because the orbital public transport network is entirely inadequate!
I'm very fond of the ELR, so I'm absolutely not saying I'd like to see this happen, but for a project like this it's not really a case of them "letting it happen" or not. A transport and works order with compulsory purchase powers would override any such opposition if approved by the relevant minister, then it would just be a matter of the amount of compensation (which might be considerable but perhaps not so much in relation to the overall cost of the project).
I agree that compulsory purchase is an option, but it would be silly to incur the cost of applying for such an order, including the inflation in construction costs whilst we wait for the necessary legal work to be done underlying the TWO.