We have tickets boarding at Rockside and want to see Brandywine Falls. Is it relatively close to Peninsula to walk there?
If you deboarded at the Boston Mills station (north of Peninsula) it would be about a two hour round trip hike (with some elevation change) to Brandywine Falls and back.
Boston Mills is mainly a bike stop so you would need to let the car attendant know so you can deboard at the bike car.
You might want to consider driving to Brandywine Falls after your train ride.
Driving to Brandywine Falls is your best bet.
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