I would say go and electrify the additional tracks. It will never happen but the electrification needs the following part of installations.
1. Build the CAT as constant tension . That will reduce the probablity of CAT getting snagged causing service outages.
2. Evidently it will need to be plate "H freight clearances.
3. Build it with capability of being 25 Kv AC CAT. That might cause the support poles to be maybe 1 - 2 feet higher ? The contact wire will need to be thicke for the lower voltage DC anyway. Support and travelers will be built for DC anyway.
4. The AC impeadance bonds on the tracks would not be needed yet but track designed for them to be added if 60 Hz ever installed.
5. Signal system built for both DC and AC much like what is used at NYP and LIRR Queens.
6. DC requires substations every 5 - 8 miles. So build a DC substation at either end of the new track. Have commercial AC power supply these substations with capability to provide 12.5 / 25 Kv AC on CAT poles to 2 or 3 DC substations between ends. Space at these intermediate provided to install in future AC paralleling stations.
7. The next order for additional cars (should be for both NICTD and the IC . Cars will have in electrical cabinet switching gear. The DC will go to an inverter for it to provide proper AC for its tractiont motors, signal gear, HVAC, lighting,etc. . If NICTD ever goes to AC CAT or just maybe part AC CAT it can install a transformer rectifer for power to the inverter.
I firmly believe that the METRA and NICTD lines will be converted to 60 HZ with in the next 20 - 25 years at the most. That will depend on when some 60 HZ electric trains start to go to several CHI stations.So when the next order for cars for the agencies is made they should be dual AC DC capable. That way as the OCS is changed to AC all the operator will need to do is throw a switch to change between the two. Much as MNRR does now for their M-8s.