I'm beginning to set up blocks with Marklin C track and want to know which rail to isolate, how to do it and how to wire it to the s88 decoder.
Should it be one ground rail with cuts in the rail or some way to do it with the isolating the hot center rail. Does the s88 decoder have to be grounded ?

Hi!

The cheap approach for three-rail layouts is separating one outer rail. This reduces ground contact of locos by half (and can be a problem with some modern model locos).
Some folks add a diode to make the rail only half-isolated. This "diode trick" improves ground contact a bit.

Others separate the centre rail and use current sensors (like two-rail layouts would use). Track voltage will be a bit lower because of the current sensors, but otherwise track contact will not be impaired.
People using that approach normally have a centre rail pickup in the last car of each train.

I run my trains manually, but I think I'd go for current sensors if I wanted automation on the layout.

I also use track isolation on C-track with a diode in each isolated section. The sensors are connected to a Loconet device developed by another member of our Marklin Modellers' Group. No problems have been encountered. I have been operating for about 6 months but other members in our Group have been operating for years using this contact method. I also use Train Controller. This feedback method is simple and effective. My blocks are divided into 3 contact sections - a short stop section at each end and the middle contact section. This enables me to stop my locomotives at the end of each block from either direction with the signals placed in the stop sections. Using this feedback method the complete block is a feedback section enabling me to determine if there is any rolling stock in a block and preventing accidents.

Mike