Correction to my earlier answer. Sheridan Rd going south of Irving splits and goes east and continues south as Sheffield. Comps are available on Sheffield south of Wrigley to Belmont.
There are a few locations in Lincoln Park and Lakeview where condo's have been built on either side of the street that'll give you a good idea of what the discount is for backing up to the El v's not. In a number of other areas the properties backing up to the El are 2 flats or rental buildings that have been held for a number of years which make it difficult to find a condo comp. As Elvis mentions in his answer, Sheridan Rd south of Wrigley and north of Belmont provides the best stretch of condos recently built that could give you a good indication of price differential from one side of the El v's the other.
Some stretches of the El are less noisy if you're close to the station and trains are moving more slowly but the bottom line is when you back up to the El you eliminate a huge part of your buyer pool. You have to price it to stand out against the competition in the area. Units sell that back up to the El and if it's priced right, shows well and is in a location that buyers can't normally afford, you'll find a buyer.
Best of luck, Ken.