A couple of small refinements to the earlier spreadsheet which hopefully might make some things clearer - with each step written out in full.
There are two versions, one
Feed Resistor.xls (Size: 21.5 KB / Downloads: 359) working out the feed resistor for a given set of parameters eg question 1 in 1997 (although that was for an ac track, but most of the principles are the same - maybe someone could post an answer for the ac track); and the other {attachment removed because it is wrong!, see post below} calculating the theoretical maximum length of the track circuit as in question 6 in 2007.
There are two versions, one
Feed Resistor.xls (Size: 21.5 KB / Downloads: 359) working out the feed resistor for a given set of parameters eg question 1 in 1997 (although that was for an ac track, but most of the principles are the same - maybe someone could post an answer for the ac track); and the other {attachment removed because it is wrong!, see post below} calculating the theoretical maximum length of the track circuit as in question 6 in 2007.