The new service failed to deliver on its promise to serve as a viable replacement due to the poor state of the rail infrastructure.

Ethanol, methanol, natural gas and electric cars are seen as viable replacement fuels to compete against oil in an open market.

However, the available tonnages and lesser quality of these materials prevent them from being a viable replacement for mined aggregates on a large scale.

Foreseeing a future when human population growth would outstrip food production and result in food shortages, their hope was to create a viable replacement for food and prevent widespread starvation.

Having long jettisoned previously acceptable - yet in many views, including mine, archaic - forms of address, we have (the argument runs) yet to develop viable replacements.