Inheriting List<T> to implement collections a bad idea?

I once read an article by Imaar Spaanjars on how to build 3 tier applications. (http://imar.spaanjaars.com/416/building-layered-web-applications-with-microsoft-aspnet-20-part-1) which has formed the basis of my coding for a while now.

Thus I implement collections as he has done, by inheriting a List<T>. So if I have a class named Employee,to implement a collection I will also have a class Employees as below.

class Employee
{
   int EmpID {get;set;}
   string EmpName {get;set;}  

}

class Employees : List<Employee>
{
   public Employees(){}
}

I never really questioned this as it did the work for me. But now that I started trying out a few things I am not sure if this is the correct approach.

e.g. if I want to get a subset from Employees, such as

 Employees newEmployees = (Employees) AllEmployees.FindAll(emp => emp.JoiningDate > DateTime.Now);

This throws a System.InvalidCastException . However, if I use the following then there is no Issue.

List<Employee> newEmployees = AllEmployees.FindAll(emp => emp.JoiningDate > DateTime.Now);

So how do I implement Employees so that I dont have to explicitly use List<Employee> in my DAL or BLL? Or maybe how do I get rid of the InvalidCastexception?