Wicked asked why they didn't build center platforms in ~1962 for the Newton stations. In that clean-room scenario, Track 1 would be pressed hard against the highway lane. A center platform has to be a little wider than a side platform so that people don't back up and fall. In Auburndale, the pike embankment would have needed at least a retaining wall. The biggest problem would have been nowhere to place the bridge pillars, so you would need more expensive bridges with overhead girders.
Besides the lack of Track 1 platforms, the MTA moved Riverside Station so they could build a big parking lot, but preventing folks from transferring with the local trains (what we now call Commuter Rail). They could have easily put an additional station at the original Riverside location with a small loop to turn the trolleys.