The next ETCS Level?

How can the railways be full if the lines are empty most of the time? That is the question the general public asks when railway companies say they cannot add additional services because the lines have reached maximum capacity; and it is a valid one. Mainline railways currently use a system of safe operation known as `absolute block', whereby trains are kept a safe distance apart using a principle that only one train may occupy a particular block at any one time. Traditionally, such systems are designed around the worst braking train on the route; meaning better braking trains are potentially braking earlier than necessary, hindering capacity. ETCS has the potential to improve capacity through separating trains based on their individual braking performance and the moving block capability of Level 3. However, even more capacity could be unlocked through a principle known as `closer running'. This concept is sometimes referred to as `relative block', where trains are separated according to their current relative speed, braking capability and location. This paper explores what such a system might look like, how it might work, and describes the potential benefits and safety, technical, operational and cultural challenges to making it a reality.