Creating Delegation token - can't create a SecurityTokenService

I'm trying to build a system working with ADFS and claims. At the moment, this is just a "toy" implementation.

I've built a very simple MVC web application, set it up using the "Identity and Access..." wizard in Visual Studio to talk to an ADFS 2.0 server, and deployed it to an IIS server. All works fine, and I can examine and list the received claims.

The next step is to build a Web API based REST service (representing back-end services that the MVC application is going to depend on), so I want to pass the credentials across to that back-end server so that it can make suitable authorization decisions.

So the first step is for me to create the delegation token (and I'll then, hopefully, work out what to do with it in terms of the HttpClient class to make the rest call). I've got this:

//We need to take the bootstrap token and create an appropriate ActAs token
var rst = new RequestSecurityToken
{
    AppliesTo = new EndpointReference("https://other-iis.example.com/Rest"),
    RequestType = RequestTypes.Issue,
    KeyType = KeyTypes.Symmetric,
    ActAs = new SecurityTokenElement(((BootstrapContext)((ClaimsIdentity)User.Identity).BootstrapContext).SecurityToken)
};

var sts = new SecurityTokenService(); //This line isn't valid
var resp = sts.Issue(System.Threading.Thread.CurrentPrincipal as ClaimsPrincipal, rst);

But, the issue is that SecurityTokenService is abstract. I can't find any types derived from this class in either System.IdentityModel nor System.IdentityModel.Services, and the above doesn't include any reference to the ADFS server which I'll obviously need to provide at some point.

Of course, I may be going down completely the wrong route also, or am just hitting a minor stumbling block and not seeing a much larger one looming in the distance, so any advice on that would be appreciated also.

I've looked at, for example,