First a regional railroad is a class II railroad..Examples of Class IIs are WSOR,WE,EJE,FEC,DME,IAIS,BPRR and other such railroads operating between $20.5 million but less than $277.7 million..FEC was a Class 1 but,was reclassified as a Class II in 1992. Why model a Class 2? There are many reasons.You see unlike a Class 3 work aday short line a Class 2 can run unit trains, intermodel and general freight trains like the Class 1 railroads..There is no need for a large roster..Depending on traffic 25-30 locomotives should suffice for a roster..This roster could consist of GP9s to SD60Ms-maybe some SD70Ms as well! The roster could be all GE as well to include some Dash 8 40Cs! It could be a mixture of both One would be hard press to find a all Alco Class 2.However,if one is modeling a freelance Class 2 could use all Alcos.For a all Alco roster I would use C420s,424s or 425s since those where the last new Alcos. To my mind a model regional railroad should have a solid traffic base such as grain,coal,ore or stone.The road should have other industries as well to include a power plant.Intermodal traffic *could* be a growing traffic base.As far as motive power that would depend on the base traffic.Say our base traffic was coal we should lean toward 6 axle units with 4 axle units being use for yard and local work.Of course run through power could show up on certain trains.During the busiest season lease power could be used as well.