Coal fired steam locomotives do produce a lot of CO2 emissions but also aerosols which has a cooling effect (global dimming), which cancels out some of the warming produced by CO2. Diesel locos probably produce less CO2 but also less aerosols that would cancel out the CO2.

Yes, steam locomotives are significantly more polluting and less efficient than modern diesel-electric locomotives. A traditional steam engine has a thermal efficiency of only about 3–6%, meaning the vast majority of the fuel's energy is lost as heat and smoke. In contrast, modern diesel engines can exceed 30–35% efficiency. From an emissions standpoint, steam engines release large volumes of particulate matter (soot), sulfur dioxide, and nitrogen oxides (NOx) directly into the atmosphere, often in a very concentrated "plume." While modern diesel engines still emit greenhouse gases and fine particulate matter (PM2.5), they are subject to strict environmental regulations, such as the EPA's "Tier 4" standards, which require advanced filtration and catalytic reduction systems. Research suggests a steam locomotive uses roughly five times more fuel than a diesel to move the same amount of tonnage, making it both a carbon-heavy and environmentally taxing mode of transport compared to contemporary rail technology.