A tractor trailer struck a bridge in Dedham causing delays to the MBTA commuter rail.Fire officials said the truck hit the East St. bridge Monday afternoon. The drivers told police they were following the GPS when they realized it took them the wrong way on East St. The top of the tractor trailer was partially sheared off. Rail inspectors are letting trains travel over the bridge at five mph. The MBTA said Franklin bound commuter rail trains are delayed in both directions.