If you only build side platforms you need 2 elevators per tunnel. One on each side of the corridor.
If you build a centre platform as well, you need 3 elevators per tunnel. One on the north side / side platform, one for the island platform and one on the south side. If you add a 4th track on the north side of that platorm, then you need a 4th elevator so people can get to the north side!
Yeah all this retaining wall is also why it's so much cheaper to build side platforms than island platforms. Express trains don't stop at Long Branch anyway and frankly it's better for the trains passing through at 150 km/h to not be directly next to a platform. I heard second-hand about an incident at Long Branch station where a passenger's backpack was hit by a Via train passing by at high speed, shattering the laptop in the bag. Thankfully there were no injuries but similar incidents have led to fatalities elsewhere.