This section essentially follows the old, dismantled railway line of the Vogelsberger Südbahn. It runs parallel between Vogelsberger Südbahnradweg and the Reichenbach. It is a partially paved path. The section towards Birstein after the two ponds is less paved and partly muddy in wet weather. There is also a lot of knots around, which makes it slippery in wet weather. A mountain bike certainly feels more comfortable on this route than on the asphalt Südbahnradweg and on top of that you have the stream in the immediate vicinity, which brings some nice places to linger.
UPDATE 03/22/2020 .: This path has now been completely graveled and paved. Although it loses a bit of flair as a result, it is definitely an alternative to the actual southern railway cycle path. In addition, there is a more continuous, easier climb on this route than on the Southern Railway Cycle Path. Definitely worth a look.