Principle (Availability): Line availability is equal to the product of the individual station availabilities.
Motivation
In coupled line, failure of any station stops of the entire line immediately. Thus, from the perspective of the line, each station is a source of failure. Mathematically, this coupled situation translates to the overall availability being the product of the individual availabilities.
Example
Consider a line with 4 stations with each station having availability (probability of being up) A = 0.95. Since all stations must be up for the line to be running, the availability of the line (probability of being up) is given by: Aline = A*A*A*A = (0..95)4 = 0.815.