Everybody seems to take for granted that the Brown line should connect to the Blue at Jefferson Park and then extend out to O'Hare, but I think this is a waste because:
(1) Running the brown line out to O'Hare doubles service on the least crowded portion of the O'Hare blue line branch, and conversely would restrict headways closer to downtown.
(2) Making enough room in the median for a portal and flying junction with the Brown Line would involve massive, extensive, and expensive freeway work and probably significant reconfiguration of the Jefferson Park Station. $$$
Better, in my opinion, would be to curve the line south at Cicero, along the Beltway Railroad ROW, and connect at Montrose instead. This gains the following:
(1) It's pretty close (1/2 mile) from Six Corners, which IMO gives it more potential for TOD than Jefferson Park
(2) It would cost less because there would be less tunneling and no expensive blue line junction to build
(3) You could connect with both the MD-N Metra line and the UP-NW Metra line (After building platforms) instead of just the UP-NW
(4) It could be extended further south along the unused Beltway Railroad ROW as the Mid City Transitway
(5) It provides a better route via transfer towards downtown for riders on the outer brown line
The downsides are:
(1) No direct brown line service to O'Hare, but is this really that important? It would be an easy ride with a transfer, and still a massive improvement over the present condition.
(2) The distance to O'Hare for brown line riders would be slightly longer
(3) This would foreclose on a potential new blue/brown track connection, but the existing blue/pink connection near the medical district should be sufficient for non-revenue moves to/from the blue line; if it ever becomes insufficient, they could always run a track through the tunnel under Block 37.
As far as implementation is concerned:
The UP-NW line would have to be elevated over the MD-N and this new Brown Line. If double stack and catenary are considered, you would have to raise UP-NW by about 30'. This could be done with a 1% grade within 3000' on the north side and about 3500' to the south. I would leave a 1200' level section on a long viaduct over Cicero, MD-N, and Montrose. The platforms would be on this viaduct. The grade to reach the new elevation would be on retained fill stretching from just south of Lawrence to just west of Keeler/Irving Park. The Kostner overpass would have to be rebuilt. If double stack is not a consideration on MD-N, you could probably go about 5' lower but Kostner would still need to be rebuilt. This would be similar in scope and cost to the Englewood Flyover project, which, while not cheap at $130m, would be chump change compared to the cost of buidling a longer tunnel and all the freeway work required for the blue/brown junction. Not to mention the benefits of a new rail/rail grade separation on two busy passenger lines.
The brown line could curve south near Cicero and emerge in a portal on the unused Beltway Railroad ROW north of Wilson. It could cross Wilson and the Kennedy on the existing railroad bridges, curve southeast on the existing UP-NW embankment, under the new UP-NW viaduct, where a station would be located directly beneath the platforms of the UP-NW. Then it would curve south along the Beltway Railroad ROW. Potentially the first phase could even include a station at Irving Park. To allow for future extension to the Mid-City Transitway corridor, it would have to climb high enough to cross over the MD-N line. Grades are not a concern for a rapid transit line; 4% grades could extend 750' on either side.
The MD-N line would retain its existing elevation, embankment, and bridges; The Mayfair platforms could probably be moved further to the north to make transfer distances shorter but this wouldn't necessarily have to occur with the initial construction.
A tunnel under the Brown and UP-NW at an elevation of about 614' would be low enough to cross under the brown/UP-NW which are about 626' and high enough to cross over southbound lanes of the Edens/Kennedy junction which are about 595' there, and connecting to the northern end of the Blue Line platform in the median. Possibly the bridge could continue across the northbound lanes to the corner of Sunnyside/Knox, giving the neighborhood a less circuitous connection to the station, if the neighborhood would allow it. (If I had one of those houses, I sure would - I would love the convenience, and if I was concerned about privacy or crime, I would just put up a fence).
I think the gas station at the NE corner of Cicero/Montrose as well as the MBS building between the tracks on the north side of Montrose would be a good spot for a headhouse/TOD complex.
Check here for a map/diagram, for the more visually oriented.
At any rate, I hope connecting at Jefferson Park isn't a foregone conclusion. This would seem to be a feasible alternative and the EIS alternatives analysis phase requires evaluating all feasible alternatives.