Porto is nearer than Madrid, of course; scenery in N Portugal is very similar to that in Galicia, and Porto itself is well worth visiting. If you want a longer journey and more mountainous scenery, Madrid would be a better choice, as you go over 2 mountain ranges. There are a couple of direct trains a day, or you could stop off for some sightseeing in say Zamora and/or Avila; also passes through Escorial.
If time is no object and you prefer coastal scenery, you could go to Ferrol and take the narrow-gauge Feve railway along the N coast of Spain, continuing on to Bordeaux for flight to Manchester. You'll need several days for this though.
The Santiago-Madrid train is the Talgo. When I have been on this in the past, it has been the old rolling-stock, which were the latest thing 30 years ago but are rather quaint now. If you're looking for local colour, you might be better off going on a succession of local trains - I have had some great experiences on Spanish trains, as people start passing the wine, but they're not the world's most rapid.
Be aware that if it is sunny, as it often is in Spain, other passengers may well draw the curtains which will rather limit your view of the scenery (true of bus as well as train). Some of the newer rolling-stock is better in this respect, as it has tinted glass and no curtains. Long-distance trains (and buses) often show films for those not interested in scenery.