Subway facilities and systems, once complete, are not considered changeable or adaptable because of the extreme costs of modifying the existing structures. An exception to this opinion is that the guide beams of self-propelled monorails can be implemented in place of the existing steel rails of the subway system without extensive modification of the tunnels, stations, or platforms.
The basic design and operational characteristics of electrically powered, steel wheeled, steel rail train systems typical of virtually all modern subway systems are not adaptable or subject to significant modification of any type without fundamental redesign. Therefore, improvements in electric power demand, performance, safety, maintenance, durability and repair are limited by basic design characteristics, and these improvements can only be upgraded at great effort and expense. Significant improvement or redesign of existing subway systems is only achievable through replacement of systems and basic rolling stock with fundamentally different technology and vehicles. The only practical replacement system technology for subway trains would be refitting with monorail guide ways in the roadbeds of subway track systems, in configurations placing guideway top running surfaces at or near the elevation of subway rails. The cylindrical shape of subway tunnels leaves the lower third to half available for location and construction of guide way beams that are generally 3-5 feet in height or depth; which should permit development and operation of monorails in the same tunnel space that subway trains previously occupied.
Integration or connection of subway stations and access facilities with ground level buildings, development and activity systems has not materialized to any significant extent, in spite of decades of urban planning and design that have attempted to establish a mutually supportive connection between subway stations and ground level development. Only when subway stations are embedded in the basement levels of major structures or developments have subways and commercial developments enhanced the function and business of one another. Subway entrance facilities integrated into commercial and residential building complexes direct passenger traffic through the localized settings, but appear to have limited impacts on surrounding neighborhoods or business districts.