By Jason Fields

When a Metro-North train heading south pulls into Riverdale Station, everyone knows where it’s going: Grand Central Terminal.

But how would Penn Station sound?

The commuter railroad is once again trotting out that alternative, announcing last week that they are “streamlining a study” looking at the impact of sending trains across the Spuyten Duyvil Amtrak bridge and straight down the West Side of Manhattan.

“Trains, power and signals are all there,” Metro-North spokeswoman Marjorie Anders said. “The major expense would be to purchase trains.”

Electric trains would be out of the running, since the available Amtrak line is specifically for diesel trains only.

Because the track already exists, there’s no requirement for a complete Environmental Impact Statement, a painstaking and lengthy process that tries to assess every aspect of a project’s effect on an area’s ecology and community. Instead, one of the main questions in the study will be how much noise will be generated by the increased traffic on the rails, Ms. Anders said.

Metro-North says it would like to go even further than simply getting passengers directly into Midtown on the West Side, creating two new stations along the way, at West 125th Street and another further south.

“We think it’s terrific. If you want to go to Fairway, bingo, you’re right there,” Ms. Anders said.

She stresses that new stations could be built quickly, pointing to the hub created near Yankee Stadium on the other side of the Bronx, which took two years, though she says, “We could start it without building a station.”

Is there a catch?

Yes, there is.

Even if trains are purchased, the environmental study is finished in 2011, as expected, and required funding — said to be in the millions — is found, there may simply be no room for it.

Penn Station is already used by Amtrak and New Jersey Transit trains, both of which jockey for the resources of the station, such as tracks, signboards and everything else. Amtrak owns the station, and therefore has the final say.

The national railroad is conducting its own capacity study at Penn Station with the results available in about 18 months, spokesman Cliff Cole said.

“As long as Metro North follows required procedures for the NEPA (National Environmental Policy Act), Amtrak will support the effort,” he said.