Ouch. Commuting to Arlington from north of DC is a rough one.
I second Wayman's recommendation to make weekend visits. It's much more common for folks in similar situations for a very good reason: time and money!
That said, if you want to be really thorough about investigating the possibility of living together while working in DC and New York, consider the following:
Considering that your girlfriend will be a student in NY and consequently may have a more flexible schedule (and willingness to sit on a train longer):
Live in Baltimore's Mount Vernon district near the train station (easy, pleasant 5 min walk) or within walking distance of Aberdeen (between Baltimore and Wilmington). You could commute to DC on MARC (Maryland's commuter rail which runs on Amtrak's tracks between Perryville, Aberdeen, Baltimore, and Washington) and continue to Arlington on the Washington Metro. If you commute out of Aberdeen, you have the added luxury of boarding select Amtrak trains there with your MARC ticket, which are faster than the MARC trains. You would save a heck of a lot of money compared to commuting from Philadelphia and the time would be much shorter, especially the closer you get to Baltimore. MARC also honors what's called a Student Advantage card (
www.studentadvantage.com). It costs about $20 a year and gives you a hefty 15% discount on all MARC tickets and 15% off of any one-way Amtrak ticket (excluding Monthly passes and 10-trip tickets and Acelas on weekdays).
On the flip side, your girlfriend could commute to New York from either Baltimore or Aberdeen. Her ticket would be more expensive than if you lived in Philly, but your combined tickets between the two of you would be cheaper since you'd be paying for a MARC monthly ticket instead of Amtrak.
Here's how the numbers break down:
Living in Philly:
You:
Monthly Amtrak ticket: $1134
Washington Metro Fees (assuming Rosslyn as a destination): approximately $74
Time: Approximately 3 hours
Your girlfriend:
Monthly Amtrak ticket (she could take commuter trains, but she would have to change trains in Trenton and it would take FOREVER): $1152
Time: Approximately 1 hour, 45 mins
Total cost: around $2360 per month
Or here's how things look if you live closer to Baltimore:
You:
Monthly MARC Student Advantage ticket cost: $212 (Aberdeen) or $148 (Baltimore)
Monthly Washington metro cost (approximate): $74
Time (very approximate, assuming Rosslyn as a destination in Arlington): Approximately 105 mins (Aberdeen) or 75 mins (Baltimore)
Your girlfriend:
Monthly Amtrak ticket cost: $1512 (from Aberdeen) or $1656 (Baltimore)
Time (approximate, assuming she's within walking distance of New York Penn Station): 2 hours, 30 mins (Aberdeen), 3 hours (Baltimore)
Total Cost: $1798 for Aberdeen or $1878 for Baltimore
So, you'd save yourself some commuting time by living closer to Baltimore (your girlfriend would have more) and somewhere around $480 to $562 per month. Also keep in mind that the apartment rental rates in Baltimore (and especially Aberdeen) tend to be cheaper than in cities like Philly.
Rafi