It is hereby declared that the improvement of railroads transporting freight or passengers within this state or between this state and other states is a public purpose in furtherance whereof the tax exemptions provided in sections 12-251 and 13b-226 to 13b-233, inclusive, may properly be granted, such exemptions to continue, however, only as long as they result in the preservation of service, or the increase thereof over present levels, or the rehabilitation and improvement of the plant and equipment used by railroad companies providing freight or passenger transportation service within this state or between this state and other states.