I think this song is about being with someone who is depressed or otherwise has some kind of psychological disturbance (your troubled mind, a goods train running through these rooms/my life), and who is also verbally abusive ("think before you speak, my darling" and "you tied these knots, now you undo them"). The protagonist is trying to empathize (I know it's hard)... but it's becoming too difficult to cope with (you bring me down too... that's something I would not do).
I think this song is about being with someone who is depressed or otherwise has some kind of psychological disturbance (your troubled mind, a goods train running through these rooms/my life), and who is also verbally abusive ("think before you speak, my darling" and "you tied these knots, now you undo them"). The protagonist is trying to empathize (I know it's hard)... but it's becoming too difficult to cope with (you bring me down too... that's something I would not do).