I think it is entirely possible that Railfan and Railroad might have filled in some blanks in the details themselves. Not to jive on them but it happens.
Metra's Capital Projects site:
http://metrarail.com/metra/en/home/abou ... jects.html" onclick="window.open(this.href);return false;
Item 4809 LOCOMOTIVE PURCHASE says
This project funds the purchase of approximately 25 new advance design diesel passenger locomotives built to Tier III EPA specifications. These locomotives will replace 25 of Metra’s oldest locomotives which would need to undergo a second life extending rehabilitation which would include bringing them up to Tier III. It is more cost effective to purchase new locomotives.
120-127 is 28 locomotives, so unless Railfan and Railroad got more detailed information, it could be any 25 locomotives that would be replaced. It could be 25 out of 150-184. The number of new locomotives purchased can also be more or less than 25, as per the wording. Since Metra seems to have barely the number of locomotives they need, I don't think they would replace 28 old with 25 new.
When does time run out before all new rebuilds have to meet tier III? I'm finding 2016:
http://www.dieselnet.com/standards/us/marine.php" onclick="window.open(this.href);return false;
That is very little time. Thinking this might factor in to Metra's plans- that tier III will be mandatory before they are able to get all remaining F40PHs and F40PHM-2s rebuilt to "-3" specs which are not tier III compliant. Also, Metra could be panicking about how smooth the funding might be for new locomotives or anything else five or ten years from now.