LONDON (Reuters) – Forecasts that British temperatures could hit a record high this week forced train operators to cancel or slow services on Monday, reawakening an age-old debate about why the British rail network struggles in the heat. As the mercury soared, Britain’s rail companies imposed speed restrictions on the network due to the risk that extreme temperatures could force the metal rails to buckle and overhead power lines to sag, meaning journeys would take longer than normal. Below is an explanation of the British rail network and why it cannot always operate at full speed and capacity…