Only bits to add to what Rob says is that you can see all of the operating sets in circuit on the Northern Line in about an hour (9 Southport,5 Ormskirk & 3 Kirkby).
There were 2 BTPs on the platform when I got there so I asked a Merseyrail employee if it was OK to get photos.... he said I should really get permission from HQ but he asked the BTPs for me and they said OK.

If you have the "saveaway ticket" go one stop up the Kirkby/Ormskirk line to Kirkdale to view the depot (with bins from the station).

If it's Mersey rail 507/508 units you need then this is the place to get them as the station is on the northern line of the network which serves Southport,Ormskirk and Kirkby, so any trains going to these stations all pass through here.When I visited I came by train from Liverpool central station where I purchased a saver card which covers all of the Mersey rail network for £4.50.As for food I got the train back to Lime street were there are various food outlets it took around 10 mins to get there.The station has a glass waiting room so you can still see the trains if the weather turns bad ,also if you smoke you dont have to go off the station to have a fag.Photography at the station is very good and when I was there the station staff were very helpfull and I had no issues with any jobs worths.