parsing a JSON array in Swift

Right now, you have something like:

if let dep = array_dep["dep"] as? NSArray {
    for var i = 0; i < dep.count; ++i {
        let add = dep[i] as! String
        LRKSFOweekendDep.append(add)
    }
}

That can be simplified to:

LRKSFOweekendDep = array_dep["dep"] as? [String]

That assumes of course, that you define LRKSFOweekendDep to be optional. If it's not optional, you can do:

LRKSFOweekendDep = array_dep["dep"] as? [String] ?? []

But, it should be optional.

In a comment, you say that there are going to be 100 of these. Rather than having a variable for each, I would have thought that you'd rather keep an array of objects. For example, consider:

struct Journey {
    let name: String
    let departures: [String]
}

Then, to parse your JSON, you could iterate through the results:

let json = try NSJSONSerialization.JSONObjectWithData(data!, options: [])

var journeys = [Journey]()

if let results = json as? [String: AnyObject], let array = results["journey"] as? [[String: AnyObject]] {
    for dictionary in array {
        for (name, departures) in dictionary {
            if let departureDictionary = departures as? [String: [AnyObject]], let departureList = departureDictionary["dep"] as? [String] {
                journeys.append(Journey(name: name, departures: departureList))
            }
        }
    }
}

Finally, I would advise against NSData(contentsOfURL:), because that's synchronous. Use NSURLSession's dataTaskWithURL, which is asynchronous. Also, if you use data! pattern, first check to make sure it's not nil. Otherwise, if data was nil for any reason outside of your control (e.g. the web server is down, internet is temporarily interrupted, etc.), the app will crash rather than handling it gracefully.

Putting that all together, you get something like:

func retrieveJourneys(completionHandler: ([Journey]?, NSError?) -> ()) {
    let task = NSURLSession.sharedSession().dataTaskWithURL(journeysURL) { data, response, error in
        guard error == nil && data != nil else {
            completionHandler(nil, error)
            return
        }

        var json: [String: AnyObject]?

        do {
            json = try NSJSONSerialization.JSONObjectWithData(data!, options: []) as? [String: AnyObject]
        } catch let parseError as NSError {
            completionHandler(nil, parseError)
        }

        var journeys = [Journey]()

        if let array = json!["journey"] as? [[String: AnyObject]] {
            for dictionary in array {
                for (name, departures) in dictionary {
                    if let departureDictionary = departures as? [String: [AnyObject]], let departureList = departureDictionary["dep"] as? [String] {
                        journeys.append(Journey(name: name, departures: departureList))
                    }
                }
            }
        }
        completionHandler(journeys, nil)
    }
    task.resume()
}

And then you'd use it like so:

var journeys: [Journey]?

override func viewDidLoad() {
    super.viewDidLoad()

    retrieveJourneys { journeys, error in
        guard error == nil && journeys != nil else {   // make sure it didn't have network problem
            print(error)
            return
        }

        dispatch_async(dispatch_get_main_queue()) {    // now update model on main queue
            self.journeys = journeys

            // and, for giggles and grins, this is how you might grab the first one and examine it:

            let someJourney = self.journeys![0]
            print(someJourney.name)
            print(someJourney.departures)
        }
    }
}

Now, the above assumes that you wanted an ordered list of journeys, sorted by the order you received them.

On the other hand, if you didn't care about the order, but wanted an efficient way to retrieve the departures associated with a given key, you might use a dictionary, instead:

func retrieveDepartures(completionHandler: ([String: [String]]?, NSError?) -> ()) {
    let task = NSURLSession.sharedSession().dataTaskWithURL(journeysURL) { data, response, error in
        guard error == nil && data != nil else {
            completionHandler(nil, error)
            return
        }

        var json: [String: AnyObject]?

        do {
            json = try NSJSONSerialization.JSONObjectWithData(data!, options: []) as? [String: AnyObject]
        } catch let parseError as NSError {
            completionHandler(nil, parseError)
        }

        var departures = [String: [String]]()

        if let array = json!["journey"] as? [[String: AnyObject]] {
            for dictionary in array {
                for (name, departureObject) in dictionary {
                    if let departureDictionary = departureObject as? [String: [AnyObject]], let departureList = departureDictionary["dep"] as? [String] {
                        departures[name] = departureList
                    }
                }
            }
        }
        completionHandler(departures, nil)
    }
    task.resume()
}

And then:

var departures: [String: [String]]?

override func viewDidLoad() {
    super.viewDidLoad()

    retrieveDepartures { departures, error in
        guard error == nil && departures != nil else {
            print(error)
            return
        }

        dispatch_async(dispatch_get_main_queue()) {
            self.departures = departures

            // and, for giggles and grins, this is how you might grab a list of departures given a particular key

            let departureTimes = self.departures!["LRKSFOweekdayDep"]
            print(departureTimes)
        }
    }
}