I've been advocating such for years. I can't believe we're spending money to build a passenger main on NS west of Porter when we have a almost all double track passenger main owned by the state about a mile north. After doing a little research (speaking to some high ranking birdies) the answer is, the South Shore needs total double track west of MC to handle Amtrak in addition to current electric trains and freight. The feds didn't want to pay for such, or at least they couldn't agree on terms. Ergo, we have the current situation. We can drop $130m to buy NS's almost-worthless empty freight line from Dearborn to Kzoo, but we can't lay 20 miles of main from Michigan City to Emerson. Go figure...
Given that the NICTD group is quite effective at running lots of trains and reinvesting in the property, it seems to me like the feds are the ones that are hard to work with in this case.
Also, if you want railroader proof that the CSS-IC routing to downtown works well, see EspeeMike's recent comments over in the Amtrak forum, Hoosier State thread. He states that, as a former engineer on the Hoosier/Cardinal, it's far faster to get downtown via the CNIC detour (GTW to Harvey, IC to downtown, SCAL to Union Station) than the usual route up the UP through Dolton.
A rebuild/redirect of the SCAL bridge might not make sense just for a few southern Illinois trains and the CNO, but all eastern trains? 5 Michigan trains, 3 east coast trains, 3 corridor trains, and CNO? That's 12 trains/day.
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