Another person was badly wounded but no party representatives or staff on board the train to West Virginia suffered any serious injuries.
House Speaker Paul Ryan was among the passengers but was unhurt, according to aides.
The collision is thought to have taken place south of Charlottesville.
White House Press Secretary Sarah Huckabee Sanders said President Donald Trump had been fully briefed on the incident and was receiving regular updates.
The party members had been on their way to their annual legislative three-day retreat at the Greenbrier resort in White Sulphur Springs, which Mr Trump and Vice President Mike Pence was set to attend.
Following the collision, Oregon congressman Greg Walden tweeted: “We’re fine, but our train hit a garbage truck. Members with medical training are assisting the drivers of the truck.”
One party adviser said the train partially derailed.
Politicians said the fatality appeared to be someone travelling in the lorry.
Oklahoma congressman Tom Cole said the vehicle had been ripped in half and that emergency services were “putting a body away”.
Missouri congresswoman Vicky Hartzler tweeted a picture of emergency services at the scene and said she heard a total of three people had been in the lorry – one who died, another who had to be extracted by firemen and a third who was able to walk away.
Other politicians tweeted they were “praying for those who were in the truck”.
Mr Cole said he felt “a tremendous jolt” at the point of impact at about 11.15am local time, almost two hours after the train left Washington.
He said the train stopped quickly after the collision before congressmen – who were doctors – got off the train to assist.
Congressman James Comer said there were about 100 Republican politicians on the train. Describing the incident, he said he jumped out of his seat when the crash took place.
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“I looked out the side of the window and then I could see a truck, just in pieces out the side of the window,” he added.
It is not clear when or if the politicians will resume their trip.