Until 2005, it was operated and owned by the BC Gas Company and transported natural gas, jet fuel and oil. It was purchased by US-company Kinder Morgan in 2005 and is since then also used to ship other oil products such as diluted bitumen from oil sands. The pipeline has already been extended in the past few years, with new pump stations added in 2007 and the Anchor Loop Expansion crossing two national parks completed in 2008. On 16 December 2013, the National Energy Board of Canada received an application from Kinder Morgan for an expansion project of the Trans Mountain pipeline system, which would include the creation of a new dual-line pipeline alongside the existing 1,150-km pipeline as well as an extension from Edmonton, AB to Burnaby, BC. The capacity of the pipeline would therefore be almost tripled, from 300,000 to a capacity of at least 890,000 barrels per day. The new line would exclusively be used to carry heavier oils such as diluted bitumen. In the past 15 years, the company Kinder Morgan has accrued a number of oil spills in the region, including four along the Trans Mountain route since 2005. Because of this history, protests soon emerged when Kinder Morgan announced its plans for the new expansion project. Local organizations have especially pointed out health risks such as airborne contamination, environmental risks such as land-based and marine spills and the vulnerability of the west coast's ecosystem. Also economical concerns, such as the risk of loss of jobs and business for the tourist industry, farming and agriculture as well as port trade and coastal industries due to future spills etc. have been at the centre of debate. Concerns were voiced by many First Nations governments in BC as well as by municipal governments (Cities of Burnaby, Vancouver and West Vancouver have passed resolutions against the Kinder Morgan pipeline) and by environmental organizations. What is more, also local communities and public opinion seem to be concerned about the risk of spills. According to a local poll, 70% of BC residents oppose the expansion project. At the moment, the project proposal is under review by the National Energy Board (NEB). The NEB's handling of the assessment process has been widely criticized as the number of people being able to express their opinion during the assessment has been very limited to a certain number of people approved by the NEB. Especially representatives of First Nation communities have been following the NEB assessment very closely, saying that they had already been adversely affected through impacts on the environment and health of local residents that they link to the tar sands fields. In a report released earlier in 2014, the United Nations affirmed Canada's First Nations' rights, stating that Canada needed indigenous consent for pipeline projects. In July 2014, a Federal Court judge had taken the decision to let the BC First Nation challenge the review process of the expansion of the Trans Mountain pipeline. If the challenge were successful, the NEB could find itself forced to restart its review process.

Opposition to this project is substantial including cities, First Nations bands, communities, environmental groups, grassroots organizations and the general public.

The land on which Kinder Morgan intended to build the expansion is the unceded traditional territory of the Tseil-Waututh, Musqueam, Sto:lo and Squamish Nations. The Save the Fraser Declaration is a document produced by the Yinka Dene Alliance and which more than 130 First Nations have become signatories.

This declaration unites and declares the First Nations’ opposition to Tar Sands projects throughout their territories, pipelines including Enbridge and Kinder Morgan, and the subsequent increase in tanker traffic. The region of Greater Vancouver is very densely populated as are a number of other cities and districts along the route, most of which have come out firmly against the project.

The scope of concerns regarding the pipeline and tankers span the terrestrial and marine environments as well as climate change and health.

One of the more articulated issues involving the marine environment is the substantial increase in tanker traffic within the Burrard Inlet.

The increased risk of spills due to the projected growth in tanker traffic is significant given the sensitivity of this ecosystem.

A diluted bitumen spill would be incredibly detrimental given its composition; it is particularly toxic to marine life.

Bitumen and diluent, the composition of which is not disclosed to the public, separate once the compound is released into the environment discharging toxic fumes into the air while the heavier bitumen sinks to the floor of marine environments.

Impacts from a spill on the terrestrial environment would likely be more localized unless it reached ground water tables.

The process by which bitumen is released from the soil contributes a significant volume of greenhouse gases to climate change.

The regions thru which the proposed pipeline would pass encompass everything from residential neighbourhoods to sensitive ecological areas.

The National Energy Board (NEB) removed both intervener oral testimony and the practice of cross-examination of witnesses from all public hearings.

The inability for interveners to give oral testimony or cross-examine witnesses has called into question the veracity of the review process.

Frequent modifications to the pipeline route without consultation has left the impression in the publics mind that public participation is being ignored outright or at the very least dissuaded.

The impacts on health from exposure to diluted bitumen is still hotly debated but in the communities surrounding Kalamazoo River citizens experienced symptoms such as headaches, nausea, dizziness, fatigue and coughs. In August, the National Energy Board ruled Kinder Morgan could conduct survey work on Burnaby mountain in Vancouver to explore an alternate route for the pipeline project via a proposed tunnel through the Mountain.

The City of Burnaby is challenging that decision in B.C.'s Court of Appeal. Then in November and December 2014, a major conflict erupted in Vancouver when opponents to the pipeline occupied and set up a camp that aimed to stop the company's pipeline exploration work on Burnaby mountain. Over 70 people were arrested after the company managed to get a court injunction against the protesters, although the charges were later dropped when it was discovered the GPS coordinates of the injunction were wrong. Kinder Morgan also brought a suit against the activists for $5.4 million for trespassing on the Burnaby Mountain Conservation Area. This was also later dropped.

On November 28, after weeks of sustained protest, the company packed up the equipment. However, drilling work has continued in other locations, along with sustained protests whenever drill sites are discovered. In late February 2015 protests continued in Coquitlam. The ongoing review process is expected to finish in mid-2015. Construction dates have been proposed for 2016 to 2017 with a proposed start date for operations set for 2017.

The project proposal includes the construction of approximately 994 km of new pipeline, the reactivation of 193 km of old pipeline, 12 new pump stations, 20 new tanks to be added and a proposed new line to carry heavier oils with capacity for transporting light crude oils. The project also includes a proposed expansion of the Westridge Marine Terminal. The current terminal accommodates five tankers per month this expansion would increase the tanker traffic to roughly 34 per month.