The British Rail Class 373 or TGV-TMST train is an electric multiple unit that operates Eurostar's high-speed rail service between Britain, France and Belgium via the Channel Tunnel. Part of the TGV family, it has a smaller cross-section to fit within the constrictive British loading gauge, was originally able to operate on the UK third rail network, and has extensive fireproofing in case of fire in the tunnel. This is both the longest——and fastest train in regular UK passenger service.