Perhaps its biggest drawback is the need for continuous cables to be laid within the tracks, expensive to install and open to damage during track maintenance. The principle difference between this system and the more modern ones being marketed today is that Seltrac uses electro-magnetic transmission of data requiring track cables, whereas radio based systems only require aerials. Seltrac is upgrading their design to use radio based transmission. Railway signalling has traditionally required a large amount of expensive hardware to be distributed all along a route which is exposed to variable climatic conditions, wear, vandalism, theft and heavy usage. Because of the widely spaced distribution, maintenance is expensive and often restricted to times when trains are not running.
Failures are difficult to locate and difficult to reach.
On metros, access is further restricted where there are tunnels and elevated sections. For these reasons, railway operators have been trying to reduce the wayside signalling equipment and so reduce maintenance costs. Reduced wayside equipment can also lead to reduced installation costs. Moving block requires less wayside equipment than fixed block systems. There is another goal much sought after by operators - greater capacity.