Both are train numbers. 1 is an actual train, the westbound Sunset Limited. 421 is an artifact of the reservation system to represent the Chicago-Los Angeles through cars carried by the Eagle (21) and the Sunset (1). It never becomes an independent train, unlike its sisters 27 and 449.
The reservation system has to have a separate number for cars traveling between different sets of terminals, even if carried on the same physical train. The Eagle used to carry a set out coach to St Louis that was represented in the reservation system by train number 821. So leaving Chicago, the Eagle on LA days consisted of "trains" 21, 421, and 821 in the reservations system. Operationally, it was one physical train, 21.
This practice can cause confusion for novice riders, on LA days an inquiry on the website for travel for points between Chicago and San Antonio, will return a choice of two different "trains", 21 and 421. They often are confused about which one to choose, the answer being the cheapest since it is the same physical train. On FB I am a member of the Empire Builder group and that question comes up all the time for 7 versus 27.