Cost effectiveness of taking train

Okay, so I enjoy riding the train to work, but haven't done it for a few weeks.  I guess it's a good thing I didn't get a February monthly pass :)

A while ago, I had decided that taking the train was a little less cost effective than driving all the way, but the benefits were worth the cost.  Well, with gas prices changing, it's time to actually calculate it.

In my case, I have to drive to a train station (or walk a mile each way, and transfer to a bus and then transfer to the train), which happens to be on the same route as driving.

M = distance driving to the office = 15 miles
M' = distance driving to the train = .2666M = 4 miles
mpg = car's average mpg = 28
Pg = price of gas per gallon
Pt = price of a daily train ticket = $4.50
Ptm = price of a montly train pass, amortized to a daily figure (given 20 days used per month) = $3.75

Cost of driving all the way:
2*M/mpg*Pg (+ neglected cost of wear on car)
1.07*Pg

Cost of driving to train station and taking train
2*M'/mpg*Pg + Pt
0.29*Pg + $4.50


Cost of driving to train station and taking train with monthly pass

2*M'/mpg*Pg + Ptm

0.29*Pg + $3.75

So, what price does gas have to be to be worth it?

1.07Pg = .29Pg + 3.75

.78Pg = 3.75

Pg = $4.81.  Wow.  Hope that doesn't happen any time soon.

EXTREME optimal case - 23 days working per month - $3.26 = Ptm, adjusting Pg to $4.18...