Can i change velocity with ClickDetector?

I am new to scripting and i am trying to build a train and use a tactic for velocity to make it move. I made this code:

local clickDetector = workspace.Red.ClickDetector

    function onMouseClick()
        game.Workspace.track_boy.Velocity = Vector3.new(-25,0,0)
        game.Workspace.track_man.Velocity = Vector3.new(-25,0,0)
    end

    game.clickDetector.MouseClick:connect(onMouseClick)

But it doesn't work.. Can someone tell me what's the problem or should i try something else?