Lionel Legacy M1a vs M1b

That's hard to say not knowing if you are really into the small details of different engine's revisions or era's.  The biggest difference will be on the front pilot and boiler front, but if you are talking about the general appearance  and non train enthusiasts just looking at it then no.  As you said the new one has whistle steam and also the IR sensor for use with the Sensor track. 

If it was me I would either sell my old one and use that towards the new one if I really had to have the few extra features or I would keep the one I had and spend my money on another totally different engine.  They are going to run around 1100.00 street price, so you could get 2 diesels for that price.

But if you really love the Pennsy Steam you could always run them both together and really pull some long freight consists!

The b's usually had boiler cleanout plugs (3) on either side of the Belpaire fire box for maintenance.  Pilot on the b was solid with in some cases a fold down or retractable coupler. b's may have had radio phone equipment but a's may also of had the antenna and radio phone add ons.   Here is a link to the one existing M1 a or b   Boiler plate shows b, Railroad Museum of Pennsylvania, Lancaster, PA.  I was fascinated with all the appliances, steam and air operated.

Ben,

If having an era specific engine is important to you, Lionel's new M1a is configured as they looked from 1930 until 1946.  Their M1b is configured like most M1a and all M1b engines were starting in 1947.  If era is not important, go with the M1a because it has the IR sensor.  You may not use it now, but you may want it in the future.

The IR is used with Lionel's LCS (Layout Control System).  It requires a Legacy base and a Sensor track to get started.  The sensor track is a 5" piece of Fastrack with an IR Receiver.  As the engine passes over the track it reads the engine information.  It also allows you to execute custom track actions and engine sequences.  They also have a set of preset options (like blowing the whistle at grade crossing automatically).  There are 8 preprogrammed options plus you can do your own customized option.  If you use the LCS iPad app it also shows engine location on your layout.  That's just a quick nutshell, but you can go on Lionel's website for detail information or the Track & Power catalog.  For legacy engines with the sensor you can just run the engine over the track and it will load all the engine info into the Cab-2 Base and Remote. (instead of loading the orange modules).  I would expect the IR sensors capabilities to be expanded as the LCS system grows.  Legacy engines from 2011 on up have them and some Vision ones before that like the Vision 700E Hudson.

I have both and love them both.  I was able to pick up the M1a for about $425 last year and I added Gunrunner John's Super Chuffer board in it which makes it act, in some ways, very much like its Legacy counterpart the M1b.  I also added 2 additional lobes to the cam to give it 4 chuffs per revolution.  

I live in PA and I have become quite the fan of the PRR after sending my Lionel FEF to get some custom work done to it.  

Thanks guys... I went ahead and ordered the Legacy M1a from Charles Ro.  Looks like they are supposed to ship soon.  Now, to get the layout underway so I can run it.

I really appreciate your help.  It has been a long time since I pulled the trigger!

Ben