// base class for all object
public class TrainingClass
{
    public double NetSurface { get; set; } = 0;
}

// one sample object
public class Blank : TrainingClass
{
    public double Height { get; set; } = 0;
    public double Width { get; set; } = 0;
}

// the handling class
public class RegModel<T> where T : TrainingClass
{
    // empty constructor
    public RegModel() { }

    // 2 methods that use data of type T
    public void Train(List<T> datas) {}
    public void Solve(T data) {}
}

// create the data type the generic need to be of
Type dataType = typeof(Blank);

// create a dummy list of data
var datas = new List<TrainingClass>();

// create the generic base type
Type genericClass = typeof(RegModel<>);

// create the generic type
Type constructedClass = genericClass.MakeGenericType(dataType);

// create the class
var rm = Activator.CreateInstance(constructedClass);

// get the train method
var trainMethod = constructedClass.GetMethod("Train");

// invoke the train method passing the List<TrainingData>
trainMethod.Invoke(rm, new[] { datas });

var trainMethodGeneric = trainMethod.MakeGenericMethod(typeof(Blank));