Although I prefer a dedicated remote (sounds like a recurring theme), I have some Amazon Fire tablets which were purchased on sale for $40-50 that are used for the DCS wi-fi app. They seem to be a good compromise size-wise - phone is too small. There's a bit of work involved with adding an app from the Google store, but I have no issues with ads or other garbage.
I haven't used them for Lionel apps yet, but hopeful that they will work. Amazon now has a generic Android tablet for under $35 - I have no idea regarding quality. The clearance MOTO G for $19 mentioned above sounds like a steal, but I wouldn't hold my breath for finding one.
Following this thread with interest, as I doubt we'll see the development of any more dedicated remotes.