How to use Rope Constraints and gradually increase and decrease values??.

So, I am creating a rollercoaster system and currentley, I am working on the chain lift system which will carry the cart up the hill. However, I have decided to use a RopeConstraint instead of CFraming it because, I want to make the coaster as realistic as possible. The part which has got me stuck is decreasing and increasing the value of the length of the constraint when needed. I intend for the coaster to be player operated but also automatic operation as well so, there is no need for players to run it. What I'm trying to accomplish is a system where the coaster can be carried up to the top and then the constraint will release the coaster as soon as it has reached the drop and go back down ready for the next cart. Also intend for this to be used if an E-Stop is to occur on the ride. I just need an idea of how I can change the value gradually and then when needed get the chain back down the hill ready for the other cart.