Two derailments within 48 hours on the London Underground (the Tube) raised questions about the systemÂ’s safety and sparked criticism of the TubeÂ’s maintenance, which was taken over by private firms last January. The incidents raised to three the number of derailments this year, the previous one occurring in January. One official said that track walks to check the rails used to be done every day but now are done once a week. Authorities vowed to investigate and offer solutions.