Lifeless monoculture lawns are as big a waste of resources as car centric infrastructure. Doubly so when it's in a place where humans can't even walk on it. Triply so when it's in a spot where it will gum up and corrode the rails it's trying to hide.
You do realize this exact system has been utilized everywhere in the world for decades without issue? The alternative is completely sealed pavement so what exactly do you propose to improve this? You wanna have the car lanes back?
Civil engineering isn't one size fits all and I'm sure there are some climates + layouts where this makes sense. But I'd prefer putting that green space somewhere where people can use it and minimize the cost, focus and footprint of transportation infrastructure as much as possible.
The two options are green or gray and depressing. The cost of maintaining it wont even show up on the chart compared to the operating cost of the tram network so that shouldnt be an issue.
The options are how you spend your money. Improving tree density can have more benefits for the same budget spend. If that's the case, keep my tram gray and let me hear more birds.
But how exactly is it better? Nicer to look at maybe, but there are other materials that can provide drainage and temperature control with better durability and less maintenance.
I can already hear the thumps from a healthy culture of critters getting splatted by the train running through wild shrubs over the rails.
Seriously, rock ballast has a maintenance cost too. Concrete has higher construction cost but is cheaper to maintain, but creates the heat island effect. Grass can still help with drainage if engineered well, removes the heat island effect, and is not too much more to maintain, since trams are a lower speed and weight class putting less of a load on the rails. Grass is even better than concrete slab for noise dampening. So grass isn't entirely purposeless and make for a pleasant scenery for people to be near.