Train travel can be delightful as a tourist or tiresome as a commuter but what is it like to experience life aboard trains that have some of the toughest jobs on the planet? Zay Harding sets out to ride with those who operate in conditions from desert furnaces to frozen wastelands. Siberia's Ice Trains are surely amongst the toughest of tough trains. Russia has the world's biggest reserves of gas so the railway is vitally important to the nation's economy as a means of exporting highly valuable condensed gas from the region. All this has to be conducted in average temperatures of minus 20 degrees centigrade. Even the snowplough trains have to work 24 hours a day to keep the lines open. On the other side of the scale, Bolivia, since the 1860s, has lost land to all its surrounding countries, leaving it land-locked and without vital access to coastal ports. As compensation, both Chile and Brazil agreed to build railways from Bolivia to their coasts, but they haven't received proper investment since. Zay attempts the tortuous and often fractured journey to the Pacific Coast. With the final leg requiring a 4,000m decent to sea level, which requires the driver to hit a dead man's button every few seconds to prove he's still conscious and forces Zay to the floor with an oxygen mask, this is tough every step of the way. The Tough Trains mantra is - keep the line open and keep the train moving.