The real issue would be designing a decent third rail electric - none currently exist anywhere in the world, and there's a big issue of wicked high currents at the shoes.
Of course, the LIRR could always take the DMs they want to stick into storage, and rip the diesel crap out of them - instant electric loco, and probbably about 30 tons lighter, so it'll perform better to boot.
You'd still need two, though.
Electric able to pull the weight? Please, even the 'too light to function' AEM-7s are used as freight motors in Sweeden* (well, the RC-4, which is slightly lighter). Once you get the train rolling, it's an HP game anyway, you can math this out in Excel, but IIRC, the F-40's absolute (i.e., theoretical) pulling advantage goes away by about 10mph. A DM-30 could do better, with AC traction and all the extra weight, but it's so low in HP in diesel mode that overall, it's not going to win.
For doing the curves, you're supposed to be able to use:
TE = 375 (e) HP / V
TE is tractive effort in lb.
e is the efficiency of the transmission (generally 85%)
HP is the rated horsepower of the engine
V is the speed in mph.
*FWIW, EMD's book says 51,500 Lbs TE, though 'maximum short time', though they don't define short time. This is about 25% adhesion. For an AC traction loco, I think 30-35% is the expected rate.
Your mileage may vary
I tried punching in the ALP-46's published data (from Caltrain's amazingly poor review of electric equipment, but scans of AdTranz's origional published specs.), and got pretty darn close (within 1 or 2 %) with the above equation...