https://jamanetwork.com/journals/jamapediatrics/fullarticle/2828425
It says fluoride levels of 2-4mg/L are correlated in a reduction of IQ. But technically the WHO has long since recommended a maximum of 1.5mg/L due to a risk of fluorosis which is a comparatively minor concern, though it's hard to find information about what levels are considered safe beyond "recommendations". Either way many places have higher than 2mg/L of fluoride in the tap water (due to natural content).
The first leg should be short and intrastate. The first step is building any high speed rail at all.
The most important thing is to keep building. Once you stop the knowledge is gone.