The state’s heavy investment in the Fairmount commuter rail line will soon be put to the economic test in a very tangible way: Two requests for proposals have been issued by the city of Boston in recent weeks to gauge interest in a pair of sites deemed as ideal for so-called “transit-oriented development.”

The properties – the old Cote Ford car dealership on Cummins Highway, and the former Maxwell Box warehouse on East Cottage Street – are considered lynchpin projects along the Fairmount corridor, long considered a potential game-changing engine for economic growth in Dorchester, Mattapan, and Hyde Park. As Boston Globe columnist Paul McMorrow wrote this week, when these properties — and others that are not yet being marketed — are “suddenly steps away from rapid transit stations, their value, and potential, will soar.” At least, that is the expectation.

The response from the city’s development community will be closely watched for both Cote Ford and Maxwell — and that’s a good thing. But, for all the optimism, it’s troubling to see that another key property — the MBTA-owned parking lot in the heart of Mattapan Square next to the trolley and bus station on River Street — is on track to be sold to a charter school this summer without the benefit of any sort of public process.

As the Reporter revealed last week in a front-page story, the MBTA has quietly been in talks to sell the 1.75 acre parking lot to the Boston Preparatory Charter School, a well-respected institution now housed in Hyde Park. According to our report, the T is poised to sell the land to the school for $1.5 million, perhaps as soon as next month. The school intends to put a new building on the site.

On the one hand, the school’s interest says something positive about the potential of the site and Mattapan Square itself. But the fact that the site has not already been secured for a mixed-use development project— one that has been envisioned for the property since at least 2004— is a huge disappointment for a business district that serves as a gateway to the city itself.

Part of the problem is that, as McMorrow correctly points out in his article, “there haven’t been many developers willing to build something of consequence on the outskirts of Uphams Corner or Mattapan Square.” In fact, the Mattapan Square site has twice been marketed with an RFP seeking a development team interested in a mix of housing and first-floor retail, based on the model of the Carruth Building next to Ashmont Station, but a 2007 request garnered only one serious bidder, who later dropped out when the economy nosedived. A second RFP, issued in 2011, netted not a single bid.

The MBTA last week told the Reporter that lawmakers’ calls for a third RFP would only come if the pending deal with the charter school — which had not been publicly revealed until last week— falls through. The T says the school would be required to hold a community process before building on the site, but that would come after the pending sale. A spokesman for the agency also noted that the T has carefully reviewed the school’s plans and finds that it will not adversely impact its station flow next door.

But the importance of this parcel goes far beyond the immediate wants and needs of the MBTA. The broader community has long viewed this lot as a potential game-changer for the square, a place that could bring in a mix of affordable housing and new retail options, including a restaurant like the ones that have enlivened Peabody Square. Beyond that, the proposed use of a school will mean that the city will lose tax revenues going forward.

The decision to site a school of any kind on this parcel should come only after a full community process is held — including a third request for proposals. There have been significant changes in the market, the civic and transportation system and, critically, in the city administration over the last three years. Mattapan deserves a chance to know what all of its options are before sanctioning the sale of this pivotal site without a more careful review. We hope that Mattapan Square’s MBTA lot will be integrated into a larger strategy by our public officials as they seek to market public-private partnerships in and around transit hubs in our communities.