Answer:

rrs

Answer	Crossword Clue
RRS	Monopoly props.
RRS	Santa Fe and others: Abbr.
RRS	Trans-Siberian and others: abbr.

RRS	Monopoly purchases: Abbr.
RRS	Monopoly quartet: Abbr.
RRS	Reading and Short Line
RRS	There are four in Monopoly
RRS	Monopoly has 4 (abbr.)
RRS	Reading et al. (abbr.)
RRS	Monopoly quartet (abbr.)
RRS	"Monopoly" quartet (abbr.)

RRS	Things with Xings
RRS	Short Line et al.
RRS	Monopoly foursome: Abbr.
RRS	Many Jay Gould holdings: Abbr.
RRS	Some Monopoly props.
RRS	Train cos.
RRS	HO-scale sets
RRS	Amtrak, et al.

RRS	They have Xings
RRS	Transp. systems
RRS	Penn Sta. users
RRS	Where to see some diners: Abbr.
RRS	Reading and others: Abbr.
RRS	Monopoly buys: Abbr.
RRS	Santa Fe et al.: Abbr.
RRS	Cos. with ampersands, often

RRS	Where to see some sleepers: Abbr.
RRS	Tourist's options: Abbr.
RRS	Travelers' options: Abbr.
RRS	Monopoly acquisitions (abbr.)
RRS	Els, e.g. (abbr.)
RRS	Reading and others (abbr.)
RRS	Central Pacific, Southern Pacific, and Union Pacific (abbr.)
RRS	Monopoly buys, briefly

RRS	Monopoly holdings (abbr.)
RRS	Penn Sta. traffic
RRS	Short Line and Reading (abbr.)
RRS	Reading and the like (abbr.)
RRS	Commuters' choices (abbr.)