Hi Matt/Ed,
Let's see if I can answer this one.
Monson #3 was the first locomotive used at MNG after the move from Massachusetts to Portland, and was the primary motive power for just about every steam day from 1993 on. Monson #3 is also the engine that goes everywhere that there is a special event, as it is far and away the lightest of the 4 steam locomotives in the MNG collection. After the FRA hammer fell on the MNG, #3 was sent to Phillips on a short term lease. Since #3 wasn't up to FRA form 4 specifications, it was decided that #3 should go somewhere that it could be run, rather than just sit around in the engine house at MNG. Hence the lease with Sandy River.
B&SR #8 made the trip also in 1993, and was used on special occasions and as the secondary steam locomotive through the end of the 2002 season. By the end of the 2002 season, #8 had developed a steam leak somewhere on the engineer's side of the running gear. That problem led to the decision to move engine #7 to Maine early the following year. When the National Narrow Gauge convention was announced for Portland, Maine in 2007, a Form 4 waiver was applied for to put #8 back in steam for that week, but was withdrawn when crown sheet ultrasounds revealed that she would only be able to be steamed to approx 95 PSI.
Monson #4 made the trip in 1993 as well, but actually sat in the museum for a number of years next to the Rangely, and was taken out of "storage" in early 2002 and brought up to steam (up to steam, get it!) early that spring. When the FRA started to oversee the steam program at MNG in the winter of 03/04, it was decided that Monson #4 would require the least amount of money to bring up to code. She was thus put through the necessarry work and came out in time for the first steam day of 2004. Since then, Monson #4 has been the only Maine 2' steam engine to have an FRA Form 4 certification.
B&SR #7 was brought up from Edaville in the winter of 02/03 and was ready to be steamed up in April. #7 saw sporadic service throughout 2003, but had to be put on the shelf when the MNG came under FRA jurisdiction. In the spring of 2007, a pledge drive was started to help pay for the work that would bring #7 up to form 4 specifications. By December of 07, it was decided that sufficient funds had been raised to begin work, and the restoration began in January 08.