You need at least 4 story tenements to make rail based public transport viable. You can go a bit less dense with buses but not much. Refer to 'Cities for a Smal

You need at least 4 story tenements to make rail based public transport viable. You can go a bit less dense with buses but not much. Refer to 'Cities for a Small Planet'. Ricky Burdet at the LSE has done quite a bit of research into this.