Ahti Kuningas, Undersecretary for Transport at the ministry, has said that according to recent data, crushed limestone taken from mining waste in will be combined with sand as an aggregate for the line foundation lower layers.

"In this way, Rail Baltics estimates up to 3.3 million tons of mining waste will be used. The exact technical solution and possible volumes are still being refined," Kuningas wrote in a letter to environment minister Rene Kokk (EKRE), which ERR had access to.

The development is in tune with the state's aim to recycle at least 40 percent of oil shale mining waste, in part to free up the space that such slag heaps take up.

"We consider it important for the state to use as much as possible of the aggregate crushed stone from oil shale mining in infrastructure projects, in order to open new quarries planned for the production of building materials," Kokk said.

However, Kuningas noted that scope for mining waste's usage in road construction was limited.

"Crushed stone is a material whose properties generally do not meet the minimum quality requirements for use in paving structures for road, and is justified and allowed only on low-traffic roads," he wrote, adding that mine waste use is often only economically viable near to the site it is produced at.

Eesti Energia says it wants to sell at least four million tons of oil shale slag.

Rail Baltic is scheduled for completion in 2026. Preliminary construction, including viaducts and deep underpinnings to cope with Estonia's often boggy ground is already underway.

Oil shale mining is concentrated in Estonia's easternmost county, Ida-Viru, and is used in electricity generation.

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