Standing water sits underneath subway tracks in almost every subway station, especially after rainfall or snowmelt, but in certain stations of the G train it actually flows merrily along. The torrent wanes and waxes with the weather outside, but it's almost always there, a constant burbling brook between the rails that runs through the stations under Marcy and Union Avenues (Myrtle-Willoughby, Flushing, and Broadway are the stations where I witness the phenonmenon firsthand).
I haven't discovered what specifics of construction or location result in this stream, or whether the water is direct runoff from streets or diverted from another part of the system. Water building up in the subway system is usually cause for alarm--unsurprisingly, high-voltage subway lines and water don't mix well, and to add insult to injury for those waiting on a flood-delayed train, water chases rats onto the platforms--but in this instance, things seem pretty well in hand. After all, the water is running somewhere and there haven't been reports of a massive urban aquifer slowly rising underneath Williamsburg. Most likely it either flows into the sewers or some kind of underground storage, or the MTA pumps it back out to street level. They might even be using it for cleaning or cooling as part of a new series of green initiatives.