Now back.in the UK but a bit more information.Took daytime trains Phitsanulok to Lopburi, and Lopburi to Bangkok Hualamphong. The double track ends at Lopburi going North but is being doubled to, and bypassing, Nakhon Sawan. The 4 coach DMU from Lopburi to Bangkok was 3rd class, 67 pence for 90 kilometres but air conditioned, stopped all stations to Ban Phachi then ran semifast. There does not seem to be container traffic on this Northern line but there is considerable cement powder and oil product flow, this freight, and the Nong Khai container trains, avoiding the Ayutthaya stretch by going the Chachoengsao to Saraburi freight line from Laem Chabang or Bangkok docks. At Rangsit southbound, the line is elevated, trains to Hualamphong then return to ground level before Bang Sue and proceed slowly over level crossings. It seems strange to me why the Hualamphong stretch was not elevated. Krung Thep Aphiwat (KTW) is not finished connection wise, has optimistic signage for high speed and airport rail links but these are not even started, and the airport line starts at Phaya Thai, runs on viaduct to Suvarnabhumi above the Eastern line (from Hualamphong), is cheap at 45 baht for the 25km , and standard gauge. I have not travelled the Eastern line, future project.
The Bangkok underground (MRT) is easy to use and quick, has bilingual signage, goes to KTW but called Bang Sue

I do not think much of the architecturally hideous concrete monstrosity that is the skytrain (BTS) (elevated railway). Neither are cheap to use, maybe 20 baht per kilometre, but avoid the traffic jams.