>>1406517There's so many places where the railway is space constrained and it would be so much easier to acquire land for one extra track than two. Land acquisitions are politically unpopular and expensive, also reduce the amount of greenery which is often beside the railways in places.
Quadriplication of note: Richmond to Burnley is quadriplicated.
Operationally, triplication can do most of the work needed to increase express service capacity without needing the whole quadruplication. You can store enough trains at the far end of the line for the start of the morning peak, run your expresses and all-stops on two tracks through to the CBD, and then have returning trains on the third track operate very minimal service, mostly express and sometimes not even taking passengers. You'll see most trains headed toward Belgrave/Lilydale in the morning peak pass through minor stations like Hawthorn but also major ones like Glenferrie. You only need say a 20 minute interval to allow for a bare bones service for the very few people travelling contrapeak, letting lots of trains get back to the end of the line or into stabling.
But you're right in that for the best operational capacity you need quadriplication, especially in sections with shared lines. That way you can have pairs like Pakenham/Cranbourne complete separated, and run all stops trains on one of the track pairs not the other.
