The Virginia Railway Express (VRE) operates a commuter rail service connecting the Northern Virginia suburbs with Alexandria and Washington, D.C. The Manasses Line starts at Broad Run and Manassas; the Fredericksburg Line starts at Fredericksburg and comes into town by way of Quantico. VRE’s pet policy allows small dogs in carriers onboard: “Only small pets are allowed, and only if carried in closed containers. Trained guide and service animals assisting persons with disabilities are permitted at all times.”
For other posts about traveling with dogs on public transit, take a look at Dog Jaunt’s handy guide!