circular railway

The centripetal force required by the trolley to turn around on the railway track is provided by friction of the wheels on the rails.

Mv^2/R = Ff
i.e. v = square-root[(Ff).R/M]
where M is the mass of the trolley
v is the speed
R is the radius of the track,
Ff is the frictional force

There is no minimum speed. If the speed is low enough, it is safe for the trollet to trun along the track. You could calculate the maximum speed v(max) by equating Ff = u.Mg, where u is the coefficient of friction and g is the acceleration due to gravity.

It is a vertical circle or horizontal circle?

It is impossible, except when there is an external agent pushing the trolley on the railway.