Exactly. In the tight confines of the subway, you'll want something more wide angle than the 50. The 35/f2 is a good choice and its not that terribly expensive. Even better would be something like a 24 or 28mm but anything faster than f2.8 gets spendy in a hurry.
I use a 17-55/f2.8 in the subway; its similar in focal range to the kit lens but its f2.8 even all the way out at 55mm so it would be a lot better underground. However, even at f2.8 I'm usually shooting at something like ISO 800 to keep the noise and shutter speeds at an acceptable level. If I could afford a wide f1.4 lens I'd buy it in a heartbeat.