Why did some passenger cars have three axles?

[Matthew T]

For a freight car, it makes sense to have an extra axle in the center so the floor doesn’t give way. But between stagecoaches on train tracks and actual bogey coaches, why were there six-wheel variants?

 

[6Gman]

For a freight car, it makes sense to have an extra axle in the center so the floor doesn’t give way. But between stagecoaches on train tracks and actual bogey coaches, why were there six-wheel variants?

I assume it was to support the increasing weight of carriages.

Note that some of the last 6-wheel vehicles in use were milk tankers, possibly because milk is denser than other liquids carried by train, such as oils.

 

[Merle Haggard]

Note that some of the last 6-wheel vehicles in use were milk tankers, possibly because milk is denser than other liquids carried by train, such as oils.

Milk tanks were either attached to passenger trains or ran in block trains at express passenger timings. Earlier milk tanks were 4 wheeled but, due to poor ride at speed, were rebuilt to 6 wheel (usually original tanks on new underframes) from the early '30s.

 

[randyrippley]

For a freight car, it makes sense to have an extra axle in the center so the floor doesn’t give way. But between stagecoaches on train tracks and actual bogey coaches, why were there six-wheel variants?

Because technology evolves, and at the time that was the best technology available