The best place in the area you specify is probably Cow Roast, between Berkhamsted and Tring but it is a fair distance from Tring station unless you have transport; you might also find something to stand on handy at this time of year as the lineside vegetation grows quite high
Best place of the lot IMO is Old Linslade north of Leighton Buzzard as there are numerous views possible from both sides of the line,
here are some examples
The overbridge just north of the junction at Ledburn isn't bad in the afternoon / evening but only for northbound trains
PS - I presume that you have your own transport as none of these locations is very close to a station!