AYER -- Three members of Massachusetts' congressional delegation are renewing the call for greater construction oversight of Pan Am Railways' rail-to-truck auto distribution yard off Willow Road.

In a letter to the chairman of the Surface Transportation Board dated Oct. 1, Rep. Niki Tsongas, a Lowell Democrat, and Sens. John Kerry and Paul Kirk, urged increased supervision to ensure the railway is sticking to environmental safeguards required for the project. The 26-acre rail yard is Pan Am's first project for its 125-acre parcel off Willow Road. The area sits atop the overlapping aquifer protection districts for Ayer and Littleton.

The Surface Transportation Board handles railroad-related disputes, reviews rail-line mergers and purchases, and rail-yard construction.

Construction forged ahead without several required environmental safeguards in place, including a stormwater-treatment device to filter sediment and other contaminants flowing across the paved lot, as well as a liner under the locomotive holding area. The company apparently has pledged to double the length of the liner to act as a tray under the length of two locomotives, instead of just one. Concerns have also been raised about the source of the pulverized concrete being trucked into the site to act as a footing before hot top was rolled out.

Pan Am is trying to complete the project this year so it can start accepting shipment of Ford vehicles for New England distribution. Pan Am and

Norfolk Southern Railways (collectively known as Pan Am Southern, or PAS) have entered into a joint venture for rail haulage between New York and Ayer.

Pan Am Railway is on probation for three years after being found guilty of covering up a more than 900-gallon diesel fuel spill at another Ayer location in 2006. A breach of construction mandates in the current project could trigger a probation violation. A probation hearing in Middlesex Superior Court is set for Oct. 15.

Now the delegation is asking the Surface Transportation Board to get involved.

"After spending the last year working with PAS and stakeholders to ensure this project has no adverse environmental impacts, it is clear that continued oversight of this project would benefit all involved," the delegation wrote. "... we encourage the STB to require documentation from PAS of compliance to all the concerns laid out by the EPA and DEP."

In the meantime, there was a strong Boston police presence around North Station yesterday, where two freshly painted navy blue Pan Am locomotives, pulling two matching luxury coach cars, accepted several passengers. Pan Am employees wearing lanyards and IDs around their necks escorted VIPs aboard the train, which left North Station at about 8:45 a.m. It's believed the whistle-stop tour is for possible investors and Ford Motor Co. executives to be wooed into using the PAS New York-to-Ayer route they've dubbed the "Patriot Corridor."

The train roared through Ayer yesterday morning less than an hour later, passing by the Willow Road site, through the downtown commuter-rail stop and continued west without breaking speed.