You say it yourself. "unit cost and fuel consumption." Don't you think fuel consumption is higher when the train is heavier?
In the economic sense train weight is about expenses and non desirables vs. income and pax benefits.
In other words: Energy consumption, CO2 emissions, air pollution, track wear offset by income from ticket sales, acceleration (shorter travel time, better rolling stock utilization).
The lighter the train car, the less bogies are needed to carry it. Train cars can share bogies, making for a more stable ride.
A reduced number of bogies contributes to the overall reduction in the weight of the train.
The lighter the train, the less wear it causes on the railhead and the less maintenance is needed; increasing the financial viability of the operation.
It also helps to keep the infrastructure in good working condition and the allowed speed high.
The lighter the train, the less energy is needed to accelerate it (relative to the income the tickets sale can generate).
The lower use of energy per paying customer-km increases financial viability of the operation.
The lighter the train, the faster it can accelerate without excessive use of energy.
The faster the acceleration and the higher the speed, the shorter the travel time.
The shorter the travel time, the better is the competitiveness of travel by railway, attracting more pax and increasing income, further increasing financial viability of the operation.
The shorter the travel time, the more money is the pax prepared to pay for the ticket, further increasing the financial viability of the operation.
The shorter the travel time, the more efficient can the use of the rolling stock be, it can make longer trips or more trips and thus generate more income, further increasing the financial viability of the operation.
The less fuel consumed, the smaller the fuel tank can be carried, further marginally reducing the train's weight.
You might intervene that this is all of marginal importance. Combined it isn't marginal at all. And passenger rail travel in Ukraine doesn't enjoy the government support, it does elsewhere across the globe. Lithuania gets support from EU funds and Belarus subsidies passenger travel so they don't feel the harsh financial constraints UZ does. UZ has to use all ways to cut expenses and become more efficient economically.
Especially in difficult conditions like the Rakhiv line.
It is much more realistic to establish a fast regional service Rakhiv - Lviv with a travel time of <4:00 with a 650 kg/seat train than with a 950 kg/seat train.
With the 2 tonnes/berth overnight trains it is close to impossible.
Don't believe me. The leading producers of rolling stock are competing on low weight. It has become an important parameter.
Try to Google "lightweight rolling stock" and you might be surprised.
I should notice that not all weight reductions are economically feasible. Electric braking or even partly battery drive demands heavy batteries but the savings of the fuel bill compensate for that.
A train doesn't become less economic efficient if a section is converted from second to first class as you seem to suggest.
That is because the higher ticket price compensate for the lower number of seats.