The celebrated Argentinean steam locomotive engineer Livio Dante Porta devised a classification for steam locomotive development as summarized below:

• First Generation Steam (FGS) - practically all past locomotives;

• Second Generation Steam (SGS) - new designs incorporating the best proven modern steam locomotive technology;

• Third Generation Steam (TGS) - totally new formats requiring considerable research and development to achieve.

A more comprehensive description is offered by Dave Warale on page 24 of his book "The Red Devil and Other Tales from the Age of Steam":

During his time at INTI Porta was not constantly involved in the day-to-day affairs of locomotive operation, and this gave him both the time and distance to develop new ideas for improved design.  Thus many of the advances in component design which were the practical outcome of Porta's engineering research stem from those years, which were highly productive.  These advances could be grouped into three categories: those which could be applied with confidence without further research, those which still had some unknowns and which therefore required further development before application but which could be used immediately if the risks associated with the unknowns were accepted, and those which definitely required further research and development work before application. Examples of these three categories were piston valve design, the cyclonic furnace, and condensing below atmospheric pressure respectively. Porta also defined three stages in the development of the steam locomotive. First Generation Steam (FGS) included all existing designs. These could be improved without major structural alterations by the application of appropriate items of the new technology, but whatever was done to them their performance would probably still have remained on the FGS level because of their inherent structural limitations. Second Generation Steam (SGS) was defined as that which could be built new using those items of the then-available technology which required little or no further research: an example is the proposed metre gauge 2-10-0 of Fig.2. Third Generation Steam (TGS) was that which would have required a large research and development effort to materialize, such as (in the author's opinion) the ACE 3000 design described in Chapter 6.3. Burning low-grade coal, modified FGS, SGS, and condensing TGS locomotives were expected by Porta to give year-round drawbar thermal efficiencies of about 10%, 15% and 25% respectively.

Note - Porta's estimated drawbar thermal efficiency of 25% for TGS is significantly higher than the figure of 16.3% that Wardale suggests on page 501 of his his book "The Red Devil and Other Tales from the Age of Steam".  Wardale makes it clear that his figure applies to non-condensing locomotives and that "higher efficiency could only be obtained by expanding the steam to sub-atmospheric pressure and low temperature by means of condensing to counter the negative effect on the cycle efficiency of the restricted inlet steam temperature as done in stationary steam plant".